Can we emulate inter-thread signal handling such that `raise()` (i.e. `kill(getpid())`) can work between threads?

[sbc100]

Emscripten doesn't generally support signal, and in particular there are no support for multiple processes.

However signals send and received with a single program, and between threads, should be possible, and least in most cases. The only situation we most likely will not be able to support is the interrupting of a busy-looping thread.

See #14883 for an initial PR that begins this work.