selftests/powerpc: Update memcmp_64 selftest for VMX implementation

This patch reworked selftest memcmp_64 so that memcmp selftest can
cover more test cases.

It adds testcases for:
- memcmp over 4K bytes size.
- s1/s2 with different/random offset on 16 bytes boundary.
- enter/exit_vmx_ops pairness.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
[mpe: Add -maltivec to fix build on some toolchains]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>