aarch64: Add support for FEAT_SME_B16B16 feature.

aarch64: Add support for FEAT_SME_B16B16 feature.

This patch adds support for SME ZA-targeting non-widening BFloat16 instructions,
under tick FEAT_SME_B16B16 and command line flag "+sme-b16b16".

FEAT_SME_B16B16 implements FEAT_SME2 and FEAT_SVE_B16B16, in accordance with that
"+sme-b16b16" enables "+sme2" and "+sve-b16b16".

Also the test files related to FEAT_SME_B16B16 are prefixed with sme-b16b16*.
eg: sme-b16b16-1.s, sme-b16b16-1.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