SA_ONSTACK signal handler flag

It appears that `reg_sig_handler` from `psw/urts/linux/sig_handler.cpp` doesn't attempt to preserve `SA_ONSTACK` flag from originally installed signal handlers.

In particular, this seems to cause troubles with Go runtime, since it expects its signal handlers be called on the dedicated signal stack. Go [documentation](https://pkg.go.dev/os/signal#hdr-Go_programs_that_use_cgo_or_SWIG) states:

> If the non-Go code installs any signal handlers, it must use the SA_ONSTACK flag with sigaction. Failing to do so is likely to cause the program to crash if the signal is received. Go programs routinely run with a limited stack, and therefore set up an alternate signal stack.