selftests/powerpc: Replace stxvx and lxvx with stxvd2x/lxvd2x

selftests/powerpc: Replace stxvx and lxvx with stxvd2x/lxvd2x

On POWER8 (ISA 2.07) lxvx and stxvx are defined to be extended mnemonics
of lxvd2x and stxvd2x. For POWER9 (ISA 3.0) the HW architects in their
infinite wisdom made lxvx and stxvx instructions in their own right.

POWER9 aware GCC will use the POWER9 instruction for lxvx and stxvx
causing these selftests to fail on POWER8. Further compounding the
issue, because of the way -mvsx works it will cause the power9
instructions to be used regardless of -mcpu=power8 to GCC or -mpower8 to
AS.

The safest way to address the problem for now is to not use the extended
mnemonic. We don't care how the CPU loads the values from memory since
the tests only performs register comparisons, so using stdvd2x/lxvd2x
does not impact the test.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Acked-by: Balbir Singh<bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>