Initial patch for emscripten / pyodide support

[mkoeppe]

On the emscripten / pyodide platform, there is limited support for POSIX signals.

• As noted in https://docs.python.org/3/library/signal.html#general-rules, in CPython on this platform, "signals are emulated and therefore behave differently. Several functions and signals are not available on these platforms."
• https://github.com/python/cpython/blob/main/Python/emscripten_signal.c
• https://github.com/python/cpython/blob/main/Modules/signalmodule.c
• https://github.com/python/cpython/blob/main/Include/internal/pycore_emscripten_signal.h

Building cysignals succeeds, but import cysignals exits immediately because functions sigaltstack and pthread_create error out.

Here we attempt to provide basic support this platform.

• This is to support https://github.com/pyodide/pyodide/pull/4407