Now that TCG supports enabling and disabling ops at runtime, it's
possible to detect the available host instructions at runtime, and
enable the corresponding ops accordingly.

Unfortunately it's not easy to probe for available instructions on
MIPS, the information is partially available in /proc/cpuinfo, and
not available in AUXV. This patch therefore probes for the instructions
by trying to execute them and by catching a possible SIGILL signal.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>