ABI for _BitInt

[kito-cheng]

_BitInt (N) is the type defined in C23, allow user to define an arbitrary-sized integer type, where N is a postive integer larger than zero.

This proposal defined the size and alignment of _BitInt, and define the unused bits as unspecified which is same as x86-64 and AArch64.

For the calling convention part, we keep unused bits as unspecified.

Ref:

• ISO/IEC WG14 N2763: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2763.pdf

• [AArch64] Rationale Document for ABI related to the C23 _BitInt type. https://github.com/ARM-software/abi-aa/tree/main/design-documents/bit-precise-types.rst

• AAPCS64 for _BitInt(N) https://github.com/ARM-software/abi-aa/commit/d6214175f73b02d546dcd29cbade6dd2d7c5a42c

• x86-64 ABI for _BitInt(N) https://gitlab.com/x86-psABIs/x86-64-ABI/-/commit/8ca45392570e96920f8a15d903d6122f6d263cd0

Fix #300

[kito-cheng]

cc @pinskia @jsm28 @workingjubilee @palmer-dabbelt @JeffreyALaw @preames @topperc

[kito-cheng]

Plan to putting more table for the code gen among 3 different options, I was too optimistic that I could have time to doing that...:( anyway will put that once I have one.