Setting arch to combined when running b2 on apple silicon generates only x86 library

[vinayakr]

When setting the command line arch=combined on a macbook pro m1 max, the library generated is x86_64 only.

Performing configuration checks

    - default address-model    : 64-bit (cached) [1]
    - default architecture     : x86 (cached) [1]
    - symlinks supported       : yes (cached)
    - has_icu builds           : no  (cached) [2]
    - lockfree boost::atomic_flag : no  (cached) [2]

[1] gcc-13
[2] gcc-13/debug/architecture-combined/link-static/python-2.7/threadapi-pthread/threading-multi/visibility-hidden

Component configuration:

    - atomic                   : not building
    - chrono                   : building
    - container                : not building
    - context                  : not building
    - contract                 : not building
    - coroutine                : not building
    - date_time                : not building
    - exception                : not building
    - fiber                    : not building
    - filesystem               : not building
    - graph                    : not building
    - graph_parallel           : not building
    - headers                  : not building
    - iostreams                : not building
    - json                     : not building
    - locale                   : not building
    - log                      : not building
    - math                     : not building
    - mpi                      : not building
    - nowide                   : not building
    - program_options          : not building
    - python                   : not building
    - random                   : not building
    - regex                    : building
    - serialization            : not building
    - stacktrace               : not building
    - system                   : not building
    - test                     : not building
    - thread                   : building
    - timer                    : not building
    - type_erasure             : not building
    - wave                     : not building

...patience...
...patience...
...found 34822 targets...
...updating 4 targets...
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_chrono.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_regex.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_atomic.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_thread.a
...updated 4 targets...
vinayakr@Vinayaks-MacBook-Pro boost % lipo -archs ../../../boostinstall/lib/libboost_regex.a
x86_64

If the architecture is set to arm, they it properly generates arm64

vinayakr@Vinayaks-MacBook-Pro boost % b2  --with-regex --with-thread --with-chrono architecture=arm address-model=64 cxxflags="-arch x86_64 -arch arm64" toolset=gcc variant=debug link=static install --prefix=/Users/vinayakr/workspace/project2_threading/ext/boostinstall
Performing configuration checks

    - default address-model    : 64-bit (cached) [1]
    - default architecture     : x86 (cached) [1]
    - symlinks supported       : yes (cached)
    - has_icu builds           : no  (cached) [2]
    - lockfree boost::atomic_flag : no  (cached) [2]

[1] gcc-13
[2] gcc-13/debug/architecture-arm/link-static/python-2.7/threadapi-pthread/threading-multi/visibility-hidden

Component configuration:

    - atomic                   : not building
    - chrono                   : building
    - container                : not building
    - context                  : not building
    - contract                 : not building
    - coroutine                : not building
    - date_time                : not building
    - exception                : not building
    - fiber                    : not building
    - filesystem               : not building
    - graph                    : not building
    - graph_parallel           : not building
    - headers                  : not building
    - iostreams                : not building
    - json                     : not building
    - locale                   : not building
    - log                      : not building
    - math                     : not building
    - mpi                      : not building
    - nowide                   : not building
    - program_options          : not building
    - python                   : not building
    - random                   : not building
    - regex                    : building
    - serialization            : not building
    - stacktrace               : not building
    - system                   : not building
    - test                     : not building
    - thread                   : building
    - timer                    : not building
    - type_erasure             : not building
    - wave                     : not building

...patience...
...patience...
...found 34822 targets...
...updating 4 targets...
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_chrono.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_atomic.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_regex.a
common.copy /Users/vinayakr/workspace/project2_threading/ext/boostinstall/lib/libboost_thread.a
...updated 4 targets...
vinayakr@Vinayaks-MacBook-Pro boost % lipo -archs ../../../boostinstall/lib/libboost_regex.a
arm64