Implement `raise` and `tgkill`

[joshtriplett]

This implements the tgkill system call, and the raise function (which sends a signal to the current thread).

I've implemented these in the runtime module for consistency, though I'd ideally like to have these available somewhere less "beware of dragons".

[sunfishcode]

Yes, raise and tgkill can go in rustix::thread, and be implemented in the libc backend as well. tgkill is cfg(linux_kernel), and on musl it looks like it'll need to go through syscall.