Re: PR28827, assertion building LLVM 9 on powerpc64le-linux-gnu

The previous patch wasn't quite correct.  The size and padding depends
on offset used in the current iteration, and if we're fudging the
offset past STUB_SHRINK_ITER then we'd better use that offset.  We
can't have plt_stub_pad using stub_sec->size as the offset.

	PR 28827
	* elf64-ppc.c (plt_stub_pad): Add stub_off param.
	(ppc_size_one_stub): Set up stub_offset to value used in this
	iteration before sizing the stub.  Adjust plt_stub_pad calls.