Add gcc-4.9 libgomp symbols requiring --plt-thread-safe for power7

Add gcc-4.9 libgomp symbols requiring --plt-thread-safe for power7

powerpc64 ld builds plt call stubs with a read barrier to provide
thread safety on lazy plt updates, necessary on multi-threaded apps
with power7 or later weakly ordered memory.  gcc-4.9 libgomp
introduced more functions that could call pthread_create, which means
we have more functions that if referenced in an executable should
cause a default of --plt-thread-safe.

bfd/
	* elf64-ppc.c (ppc64_elf_size_stubs): Add gcc-4.9 libgomp functions
	to thread_starter.
gold/
	* powerpc.cc (do_relax): Add gcc-4.9 libgomp functions to
	thread_starter.