[PowerPC] undef Relocation names in PowerPC*.def

glibc's PowerPC /usr/include/asm/sigcontext.h, has this:

  #ifdef __powerpc64__
  #include <asm/elf.h>
  #endif

and that contains defines of all of the relocation symbols, like this:

  #define R_PPC_NONE              0

and if that file is included prior to including
include/llvm/Support/ELFRelocs/PowerPC*.def, which we cannot in general
prevent, the result will fail.

As it turns out, this happens when compiling
lld/unittests/DriverTests/GnuLdDriverTest.cpp under PPC64/Linux, because:

  lld/include/lld/ReaderWriter/ELFLinkingContext.h includes
  lld/unittests/DriverTests/DriverTest.h which includes
  utils/unittest/googletest/include/gtest/gtest.h which includes
  utils/unittest/googletest/include/gtest/internal/gtest-internal.h which includes
  /usr/include/sys/wait.h which includes
  /usr/include/signal.h which includes
  /usr/include/bits/sigcontext.h which includes
  /usr/include/asm/sigcontext.h which includes
  /usr/include/asm/elf.h

the test could be fixed to include ReaderWriter/ELFLinkingContext.h before
including unittests/DriverTests/DriverTest.h, but dealing with this in the
*.def files is a more-general solution that localizes the fix to the headers
instead of requiring changes to an unbounded number of other source files (both
in-tree and external).

llvm-svn: 248957