PR 19264 looping in ppc64_elf_size_stubs

b3991020 fixed the testcase in this PR but it may be possible to
trigger the problem in other ways.

	PR ld/19264
	* elf64-ppc.c (STUB_SHRINK_ITER): Define.
	(ppc64_elf_size_stubs): Exit stub sizing loop past STUB_SHRINK_ITER
	if shrinking stubs.
	(ppc64_elf_size_stubs): Adjust to suit.