The current runtime attempts to forward signals generated by non-Go
code to the original signal handler.  If it can't call the original
handler directly, it currently attempts to re-raise the signal after
resetting the handler.  In this case, the original context is lost.

This fix prevents that problem by simply returning from the go signal
handler after resetting the original handler.  It only does this when
the original handler is the system default handler, which in all cases
is known to not recover.  The signal is not reset, so it is retriggered
and the original handler takes over with the proper context.

Fixes #14899

Change-Id: Ib1c19dfa4b50d9732d7a453de3784c8141e1cbb3
Reviewed-on: https://go-review.googlesource.com/21006Reviewed-by: Ian Lance Taylor <iant@golang.org>