powerpc: Fix PPC64/POWER7 conform tests

When building with --disable-multi-arch the memmove and strstr POWER7
optimization create and uses symbols that conflict with expect conform
tests.

	* sysdeps/powerpc/powerpc64/power7/memmove.S (bcopy): Changing to
	__bcopy and add a weak_alias to bcopy.
	* sysdeps/powerpc/powerpc64/power7/strstr.S (strstr): Use __strnlen
	for static build.