[ELF][PPC] Improve error message for unknown relocations

Like rLLD354040.

Previously, for unrecognized relocation types, in -no-pie mode:

  foo.o: unrecognized reloc 256

In -pie/-shared mode:

  error: can't create dynamic relocation R_PPC_xxx against symbol: yyy in readonly segment

llvm-svn: 368964