[TargetMachine][CodeGenModule] Delete unneeded ppc32 special case from shouldAssumeDSOLocal

PPCMCInstLower does not actually call shouldAssumeDSOLocal for ppc32 so this is dead.
Actually Clang ppc32 does produce a pair of absolute relocations which match GCC.

This also fixes a comment (R_PPC_COPY and R_PPC64_COPY do exist).