Document Argument passing for _BitInt types

[pinskia]

Forwarded from https://hachyderm.io/@kaylie@blahaj.zone/113574803870036780 .

The current BasicCABI.md does not document how _BitInt types are passed and it would be good to document it as such.

Note if someone does a GCC backend for WebAssembly, GCC folks in general has decided the ABI for _BitInt needs to be documented before it having it implemented too.

See ARM-software/abi-aa#175, riscv-non-isa/riscv-elf-psabi-doc#300 and https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/32 for examples on how other ABIs (have or will have document it).

[sunfishcode]

I've now filed #240 to document _BitInt and a few other types.