There are three OSes that I know something about that do PowerPC assembler:
AIX, OS X, and Linux. All three of these OSes use the same ABI for 64-bit
assembly (64-bit PowerPC ELF ABI Supplement 1.7). For 32 bits, the ABIs
for AIX and OS X are essentially the same,
but Linux differs very slightly in register usage and substantially in
the way the stack is defined.
The standard ATLAS include file defines the macros ATL_AS_OSX_PPC,
ATL_AS_AIX_PPC and ATL_GAS_LINUX_PPC, which can be used like:
#error "This kernel requires OS X PPC assembler!"
to guard against invalid compilation.