[builtins] Canonicalize PowerPC detection macros to __powerpc__

The lowercase __ppc__ is not defined by Linux GCC, therefore it lures
users to write code which is not portable to GCC. Migrate to __powerpc__ in
preparation for undefining __ppc__. __powerpc__ is much more common than
__PPC__.

GitOrigin-RevId: 0f91ec243d6ddb1f8b1f80d15f9003e5f06b0a21
2 files changed