powerpc: Provide __tls_get_addr () in static libc

Since '--no-tls-optimize' is available for Power in ld, we need to provide
__tls_get_addr () in static libc in order to avoid undefined references to this
symbol when that flag is used.

	* sysdeps/powerpc/libc-tls.c: New file. Provides __tls_get_addr () in
	static libc.