PR20886, looping in ppc64_elf_size_stubs

PR20886, looping in ppc64_elf_size_stubs

The PR20886 binary is large enough that there are two stub sections
servicing .text (which is 88M).  It so happens that between one
iteration of sizing and the next that one stub section grows while
the other shrinks.  Since one section is always growing, the loop
never terminates.

This patch changes the algorithm to not update previous size on
shrinking, once we go past a certain number of iterations.

	PR ld/20886
	* elf64-ppc.c (ppc64_elf_size_stubs): Make rawsize max size seen
	on any pass past STUB_SHRINK_ITER.