Take a look at the darwin.jam file which does a lot of overriding and removal of options as it inherits from gcc.jam. The key code is:
toolset.inherit-flags darwin : gcc
: <runtime-link>static
<architecture>arm/<address-model>32
<architecture>arm/<address-model>64
<architecture>arm/<instruction-set>
<architecture>x86/<address-model>32
<architecture>x86/<address-model>64
<architecture>x86/<instruction-set>
<architecture>power/<address-model>32
<architecture>power/<address-model>64
<architecture>power/<instruction-set> ;