This commit changes signal handling in a few ways:
* It stubs signals for all wasm targets (not just wasi) and baremetal,
since none of those have traditional POSIX signals. And moves the
code for that into a single file, instead of duplicating it.
* It removes the stub for signal_ignored since the value `false` might
be wrong in some cases and it doesn't usually seem to be called (it
is not called in tsgo). Should be trivial to re-add if it is shown
to be needed.
* It adds a stub for `os/signal.signalWaitUntilIdle` which _is_ called
by tsgo.
Ayke van Laethem