PowerPC, use size_t rather than long for indices

PowerPC, use size_t rather than long for indices

This is fussing about nothing really but since I was looking at signed
vs. unsigned issues, I decided to use the correct types here.

	* elf32-ppc.c (ppc_elf_get_synthetic_symtab): Use size_t for vars.
	* elf64-ppc.c (sym_exists_at): Use size_t for lo, hi and mid.