loongarch: add missing getcontext

[yxd-ym]

to make it work for zig-bootstrap for loongarch64-linux-musl

[andrewrk]

TODO? come on... how about actually doing it.

[alexrp]

Note that it was likely copied from the riscv arch bits file which also has it as a TODO. 🙂 (riscv and loongarch share like 90% of Linux ABI details.) Even defining it to {} is actually quite a bit better than nothing because the standard library will not build at all if it isn't defined. Defining it is also surprisingly non-trivial because there are glibc ABI compatibility issues to consider.

So fwiw, I think it's fine to merge this as-is. And in any case, one of my local branches has ongoing work to thoroughly fill out the arch bits files for all Linux architectures, so I'll get to it eventually even if this PR doesn't.

[andrewrk]

OK