powerpc: Fix strstr/power7 build

This patch fixes the strstr build with --disable-multi-arch option.
The optimization calls the __strstr_ppc symbol, which always build
for multiarch config but not if it is disable.  This patch fixes it
by adding the default C implementation object with the expected
symbol name.

	* sysdeps/powerpc/powerpc64/power7/Makefile [$(subdir) = string]
	(sysdep_routines): Add strstr-ppc64.
	* sysdeps/powerpc/powerpc64/power7/strstr-ppc64.c: New file.