Define BFloat16 intrinsics

[eopXD]

NOTE: This PR currently is based upon #292

NOTE: These new intrinsics introduced are not part of the to-be-frozen v1.0 intrinsics specification. These new intrinsics will not be cherry-picked to the v1.0.x branch.

This pull request defines intrinsics for vector bfloat16 type manipulations and vector instructions that are introduced by the RISC-V BFloat16 instruction set extensions (zvfbfmin and zvfbfwma) [0].

Intrinsics introduced for vector bfloat16 type manipulation:

• (Non-segment/Segment) loads and stores around the vector bfloat16 types.
• Utility intrinsics like vcreate, vundefined, ... etc (you may find the exact list under generated prototypes and its declarations under bfloat16_inst.py)

New intrinsics are added to expose new instructions introduced:

• vfwmaccbf16.vv vd, vs1, vs2, vm
• vfwmaccbf16.vf vd, rs1, vs2, vm
• vfncvtbf16.f.f.w vd, vs2, vm
• vfwcvtbf16.f.f.v vd, vs2, vm

[0] https://github.com/riscv/riscv-bfloat16

[dzaima]

Shouldn't __riscv_vreinterpret_v_ui16m1_bf16m1 be __riscv_vreinterpret_v_u16m1_bf16m1 (i.e. u16 instead of ui16) as it is in all current vuint16m1_t type abbreviations?

[kito-cheng]

https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#cc-type-sizes-and-alignments

__bf16 has used in psABI, but we may also need to add that to c-api-doc https://github.com/riscv-non-isa/riscv-c-api-doc/blob/master/riscv-c-api.md for more complete documentation for that type.

[eopXD]

Rebase and addressed comments from @dzaima, thank you for pointing this out. @kito-cheng I'll leave it to you to add some descriptions somewhere in the C api document and we'll then cite them here.

[4vtomat]

I'll take over and proceed this patch!