linux-user: Proper exit code for uncaught signals

linux-user: Proper exit code for uncaught signals

The proper exit code for dieing from an uncaught signal is -<signal>.
The kernel doesn't allow exit() or _exit() to pass a negative value.
To get the proper exit code we need to actually die from an uncaught
signal.

A default signal handler is installed, we send ourself a signal
and we wait for it to arrive.

Patch originates from Scratchbox

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>