Return from hardware exception signal handler before processing an exception on Unix

[janvorli]

We currently never return from SIGSEGV and other hardware exception handlers on Unix. We process the exception and resume execution somewhere. This is problematic on some Unixes like Illumos where not returning from the handler skips some of the bookkeeping stuff that the return from the handler does. For example, the alternate stack is not used after a first invocation of the SIGSEGV handler on Illumos (and if we ever moved to signals for handling hardware exceptions on macOS, it has similar problem).

Consider changing the hardware exception handling to return from the signal handler redirected to a helper function instead.