PowerPC64 .branch_lt size change leads to "stubs don't match calculated size"

https://bugzilla.redhat.com/show_bug.cgi?id=1523457

I haven't analyzed this myself, I'm relying on Nick's excellent
analysis.  What I believe is happening is that after some number of
stub sizing iterations, a long-branch stub needs to be converted to a
plt-branch, but either due to stub alignment or other stubs shrinking
in size, the stub group section size doesn't change.

That means we exit from ppc64_elf_size_stubs after sizing with an
incorrect layout, in fact the additional .branch_lt entry overlays
.got!  Since .TOC. is normally set to .got + 0x8000 the stub sizing
code decides that entry is within +/-32k of the TOC pointer and so a
three insn stub is sufficient.  When we come to build the stubs using
a correct non-overlaying layout, a four insn plt-branch stub is
generated and the stub group size doesn't match that calculated
earlier.

	* elf64-ppc.c (ppc64_elf_size_stubs): Iterate sizing when
	.branch_lt changes size.