ABI for BitInt

[xry111]

BitInt has been added to C23 and GCC already supports it for x86 and AArch64, thus we can expect some real software starts to use it soon. To support it we need to define the ABI, and then we can add the support to the compilers (namely GCC and Clang).

See the RISC-V ticket riscv-non-isa/riscv-elf-psabi-doc#300 for some references and insights.