[PowerPC64] Don't define __GI_ variant of isnan for static lib

It seems to me that libc.a should not contain any of the __GI_
symbols, and certainly --enable-multi-arch ought to not add to the
list.  At the end of this patch series we have the following in both
--enable-multi-arch and --disable-multi-arch libc.a:
0000000000000000 T __GI___readdir64
0000000000000000 T __GI___fxstatat64
0000000000000000 T __GI_getrlimit
0000000000000000 T __GI___getrlimit

	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_isnan-ppc64.S (hidden_def):
	Redefine only when SHARED.