[ELF] #undef PPC to support GCC powerpc32 build

GCC's powerpc32 port predefines `PPC` as a macro in GNU C++ mode in some configurations (Linux,
FreeBSD, and some others. See `builtin_define_std ("PPC"); ` in gcc/config/rs6000).

```
% powerpc-linux-gnu-g++ -E -dM -xc++ /dev/null -o - | grep -w PPC
#define PPC 1
```

Fixes https://bugs.gentoo.org/829599

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D116017