[PowerPC64] Tidy strcasecmp_l-power7.S symbols

The routine being assembled here is strcasecmp_l, so ask for that via
__STRCMP and STRCMP defines.  That change means tweaking the power7
override.  Needed for later powerpc64le changes where we want the base
symbols, not just a power7 variant.

	* sysdeps/powerpc/powerpc64/multiarch/strcasecmp_l-power7.S:
	(__STRCMP, STRCMP, __strcasecmp_l): Define.
	(__strcasecmp): Don't define.