Defining __powerpc__ in rs6000.h

[David Edelsohn]

	The AIX compilers do not introduce any macro definitions like
__powerpc__ into the namespace, so GCC should not polute the namespace on
AIX. 

	However, CPP_CPU_SPEC and ASM_CPU_SPEC in rs6000.h clearly are
further hold-overs from AIX support.  I have no objection to moving those
definitions to aix.h as overrides and defining a more standard set in
rs6000.h.  I suspect the AIX macro definitions need to remain for backward
compatibility, but Franz and Geoff are the ones with more experience in
that area.  Adding __powerpc__ or __ppc__ to the non-AIX case would be
fine with me.

	There are many more things that should move from rs6000.h to
aix.h.  And because of the forthcoming 64-bit PowerPC SVR4/ELF ABI which
is based on the AIX 64-bit PowerPC ABI, xcoff.h eventually will be split
out from aix.h.

David