Support building for aarch64-apple-darwin target

[repi]

When building for the new aarch64-apple-darwin target for the new Mac M1 ARM CPUs with Nightly the C++ compilation fails with:

  running: "cc" "-O0" "-ffunction-sections" "-fdata-sections" "-fPIC" "-g" "-fno-omit-frame-pointer" "-arch" "arm64" "-I" "mimalloc/include" "-I" "mimalloc/src" "-std=gnu11" "-Wall" "-Wextra" "-Wno-unknown-pragmas" "-ftls-model=initial-exec" "-Wno-invalid-memory-model" "-fstack-protector-all" "-fno-strict-aliasing" "-DMI_STATIC_LIB" "-DMI_DEBUG=3" "-DMI_SECURE=2" "-DMI_VERBOSE=0" "-DMI_STAT=0" "-o" "/Users/repi/git/rust/mimallocator/target/debug/build/mimalloc-sys-4a112146a3fd8379/out/mimalloc/src/static.o" "-c" "mimalloc/src/static.c"
  cargo:warning=warning: unknown warning option '-Wno-invalid-memory-model'; did you mean '-Wno-invalid-pp-token'? [-Wunknown-warning-option]
  cargo:warning=In file included from mimalloc/src/static.c:10:
  cargo:warning=mimalloc/include/mimalloc-internal.h:353:11: error: unknown token in expression
  cargo:warning=  __asm__("movq %%gs:0, %0" : "=r" (tid) : : );  // x86_64 macOS uses GS
  cargo:warning=          ^
  cargo:warning=<inline asm>:1:7: note: instantiated into assembly here
  cargo:warning=        movq %gs:0, x8
  cargo:warning=             ^
  cargo:warning=In file included from mimalloc/src/static.c:10:
  cargo:warning=mimalloc/include/mimalloc-internal.h:353:11: error: invalid operand
  cargo:warning=  __asm__("movq %%gs:0, %0" : "=r" (tid) : : );  // x86_64 macOS uses GS
  cargo:warning=          ^
  cargo:warning=<inline asm>:1:7: note: instantiated into assembly here
  cargo:warning=        movq %gs:0, x8
  cargo:warning=             ^
  cargo:warning=1 warning and 2 errors generated.
  exit code: 1

This is something we would like to support and probably would need an upgrade of latest mimalloc library and some smaller tweaks to get working.

Related: #9