[Flang] Add __powerpc__ macro to set c_intmax_t to c_int64_t rather than...

[Flang] Add __powerpc__ macro to set c_intmax_t to c_int64_t rather than c_int128_t as PowerPC only supports up to c_int64_t. (#81222)

PowerPC only supports up to `c_int64_t`. Add macro `__powerpc__` and
preprocess it for setting `c_intmax_t` in `iso_c_binding` intrinsic
module.