tests/tcg/aarch64: add system test for FEAT_XS

tests/tcg/aarch64: add system test for FEAT_XS

Add system test to make sure FEAT_XS is enabled for max cpu emulation
and that QEMU doesn't crash when encountering an NXS instruction
variant.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20241211144440.2700268-7-peter.maydell@linaro.org
[PMM: In ISAR field test, mask with 0xf, not 0xff; use < rather
 than an equality test to follow the standard ID register field
 check guidelines]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>