[ELF] #undef PPC to support GCC powerpc32 build

[MaskRay]

Summary

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

[thesamesam]

Thanks. I don't see where PPC is reserved and I'm not even sure why GCC is setting it, but it is what it is, so this is the easiest way forward.

lld/ELF/Arch/PPC.cpp

 #include "llvm/Support/Endian.h"
 
 using namespace llvm;
 using namespace llvm::support::endian;
 using namespace llvm::ELF;
 using namespace lld;
 using namespace lld::elf;
 
+// Undefine the macro predefined by GCC powerpc32.
+#undef PPC
+
 namespace {
 class PPC final : public TargetInfo {
 public:
   PPC();
   RelExpr getRelExpr(RelType type, const Symbol &s,
                      const uint8_t *loc) const override;
   RelType getDynRel(RelType type) const override;
   void writeGotHeader(uint8_t *buf) const override;