Default powerpc64 to --plt-thread-safe for Go

Default powerpc64 to --plt-thread-safe for Go

Go is multi-threaded, so use thread-safe plt stubs.  __go_go doesn't
actually start threads, but is a convenient libgo function with a
reasonably unique name that is called by all Go executables.  (Threads
are started by a static function of libgo, runtime_newosproc, called
by a whole lot of global functions, including runtime_starttheworld
and runtime_main.)

bfd/
	* elf64-ppc.c (ppc64_elf_size_stubs): Add __go_go to thread_starters.
gold/
	* powerpc.cc (Target_powerpc::do_relax): Add __go_go to thread_starters.