Build -Os and -O2 versions of every target library

[keith-packard]

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This uses a new gcc config option, --enable-multilib-space, which duplicates every multilib configuration, adding a new version which optimizes for space. With this, every library in the toolchain will get built twice and will be selected depending on the options applied during the link phase. Add -Os or -Oz and you will get space-optimized versions of every toolchain library.

crosstool-ng PR: ~~https://github.com/zephyrproject-rtos/crosstool-ng/pull/26~~ GCC PR: https://github.com/zephyrproject-rtos/gcc/pull/39 Picolibc PR: TBD