powerpc64le: Fix ifunc selection for memset, memmove, bzero and bcopy

The hwcap2 check for the aforementioned functions should check for
both PPC_FEATURE2_ARCH_3_1 and PPC_FEATURE2_HAS_ISEL but was
mistakenly checking for any one of them, enabling isa 3.1 version of
the functions in incompatible processors, like POWER8.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>