bfd/ELF/PPC: make ppc_build_one_stub()'s stub_str[] static

There's no reason to have the compiler materialize objects onto the
stack.

In fact we can save some space and a level of indirection (and hence
relocation entries in the final binary) by converting to an array of
char[12] or larger. Pick char[16] for easier / faster calculations.

bfd/elf64-ppc.c

@@ -12226,7 +12226,7 @@ ppc_build_one_stub (struct bfd_hash_entry *gen_entry, void *in_arg)
       struct elf_link_hash_entry *h;
       size_t len1, len2;
       char *name;
-      const char *const stub_str[] = { "long_branch",
+      static const char stub_str[][16] = { "long_branch",
 					   "plt_branch",
 					   "plt_call" };