aarch64: Add support for FEAT_SVE_B16B16 feature.

aarch64: Add support for FEAT_SVE_B16B16 feature.

In the current code, SVE2 Bfloat16 instructions are implemented with tick
FEAT_B16B16 and command line flag "+b16b16" and this feature was suspended
due to incomplete support.

In the new spec available here[1], FEAT_B16B16 is replaced with
FEAT_SVE_B16B16 and command line flag "+b16b16" is replace with "sve-b16b16".

Also the test files related to FEAT_SVE_B16B16 are prefixed with sve-b16b16*.
eg: sve-b16b16-sve2-1.s, sve-b16b16-sve2-1.d.

This patch supports the SVE Z-targeting non-widening BFloat16 instructions
with command line flag "+sve-b16b16+sve2".
[1]: https://developer.arm.com/documentation/ddi0602/2024-06/SVE-Instructions?lang=en