* [Feat] Add BF16 when the machine is supported.

Finished: vld1_bf16_x4 and vld1q_bf16_x2

* [NEON] Add a C implementation of the bf16 type

* [NEON] Add all ld_*_bf16 intrinsics.

* [NEON] Add all st*_bf16 intrinsics.

* [Test] Add vbfdot_f32 test case

* [NEON] Complete converting function from float32 to bfloat16.

- Also add bf-related functions in three series
- cvt, dot, dot_lane

* [Feat] Add option '+bf16' in cross-file

* [NEON] Completed initial implementation of bf-16 related intrinsics.

* [Fix] Remove redundant commment

* [Fix] Correct native aliases

* [Fix] The test generation code has been completed.