x86emul: add high register S/G test cases

x86emul: add high register S/G test cases

In order to verify that in particular the index register decoding works
correctly in the S/G emulation paths, add dedicated (64-bit only) cases
disallowing the compiler to use the lower registers. Other than in the
generic SIMD case, where occasional uses of %xmm or %ymm registers in
generated code cause various internal compiler errors when disallowing
use of all of the lower 16 registers (apparently due to insn templates
trying to use AVX2 encodings), doing so here in the AVX512F case looks
to be fine.

While the main goal here is the AVX512F case, add an AVX2 variant as
well.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>