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

When setting the command line arch=combined on a macbook pro m1 max, the library generated is x86_64 only.
```vinayakr@Vinayaks-MacBook-Pro boost % b2 --with-regex --with-thread --with-chrono architecture=combined 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-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
```