linux-user: Remap guest SIGABRT

Distinguish host SIGABRT from guest SIGABRT by mapping
the guest signal onto one of the host RT signals.

This prevents a cycle by which a host assertion failure
is caught and handled by host_signal_handler, queued for
the guest, and then we attempt to continue past the
host abort.  What happens next depends on the host libc,
but is neither good nor helpful.

Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>