aarch64: Add support for FEAT_SVE_B16B16 min and max instructions.

aarch64: Add support for FEAT_SVE_B16B16 min and max instructions.

This patch adds support for SME Z-targeting multi-vector non-widening
BFloat16 instructions, under tick FEAT_SVE_B16B16 and command line flag
"+sve-b16b16+sme2".

Also the test files related to FEAT_SVE_B16B16 (+sme2) are prefixed with
sve-b16b16-sme2*.
eg: sve-b16b16-sme2-1.s, sve-b16b16-sme2-1.d.

The spec for this feature and instructions is availabe here [1]:
[1]: https://developer.arm.com/documentation/ddi0602/2024-06/SME-Instructions?lang=en