PR24355, segmentation fault in function called from ppc_finish_symbols

Backport to 2.30, where the problem might be triggered by gc-sections
rather than inline PLT optimization as in the PR.

	PR 24355
	* elf32-ppc.c (ppc_elf_relocate_section): Don't call write_glink_stub
	for local iplt syms with ent->plt.offset == -1.  Remove ineffective
	attempt at writing glink stubs only once.  Correct plt.offset mask.