[PowerPC] Improve consistency in use of __ppc__, __powerpc__, etc.

Both GCC and LLVM will implicitly define __ppc__ and __powerpc__ for
all PowerPC targets, whether 32- or 64-bit.  They will both implicitly
define __ppc64__ and __powerpc64__ for 64-bit PowerPC targets, and not
for 32-bit targets.  We cannot be sure that all other possible
compilers used to compile Clang/LLVM define both __ppc__ and
__powerpc__, for example, so it is best to check for both when relying
on either inside the Clang/LLVM code base.

This patch makes sure we always check for both variants.  In addition,
it fixes one unnecessary check in lib/Target/PowerPC/PPCJITInfo.cpp.
(At least one of __ppc__ and __powerpc__ should always be defined when
compiling for a PowerPC target, no matter which compiler is used, so
testing for them is unnecessary.)

There are some places in the compiler that check for other variants,
like __POWERPC__ and _POWER, and I have left those in place.  There is
no need to add them elsewhere.  This seems to be in Apple-specific
code, and I won't take a chance on breaking it.

There is no intended change in behavior; thus, no test cases are
added.