PowerPC64: align plt_branch stubs

plt_branch stubs are similar to plt_call stubs in that they branch
via bctr.  Align them too.

bfd/
	* elf64-ppc.c (ppc_size_one_stub): Align plt_branch stubs as for
	plt_call stubs.
ld/
	* testsuite/ld-powerpc/elfv2exe.d: Adjust for plt_branch changes.
	* testsuite/ld-powerpc/notoc.d: Likewise.
	* testsuite/ld-powerpc/notoc.wf: Likewise.
	* testsuite/ld-powerpc/notoc3.d: Likewise.
	* testsuite/ld-powerpc/pr23937.d: Likewise.