README: clarify hardware support and fallback mechanism + more exact origin of assembly code

[matu3ba]

First of, this looks very nice.

Luajit does not support all targets, so it would be nice to have the supported platforms as matrix, ideally in section Features.

Further more, it would be nice to have the exact commit hashes the assembly was created from.

[tidwall]

Platform	Assembly Method	Fallback
Linux	ARM/ARM64/x86_64/rv64	ucontext
Mac	ARM64/x86_64	ucontext
BSD	ARM64/x86_64	ucontext
Windows	x86_64	N/A
WebAssembly	N/A	Asyncify

The assembly code in Neco is from the embedded packages https://github.com/tidwall/sco and https://github.com/tidwall/llco. Those packages use assembly from the minicoro library (MIT-0), specifically https://github.com/edubart/minicoro/commit/ff5321d93fe2a3fb067a8dd97a37dd623337b9c0. The minicoro assembly implementation is inspired by Lua Coco by Mike Pall.

[calvin2021y]

what about ios/android support ?

[tidwall]

@calvin2021y I haven’t tested on those platforms. It may work with minimal tweaking, since both have posix apis that are similar to Mac and Linux.