tests/tcg/hexagon: Handle SIGILL internally in invalid-slots test

Rewrite invalid-slots.c to catch and verify SIGILL using a sigaction
handler that modifies the ucontext, matching the pattern used by
invalid-encoding.c.

Reviewed-by: Taylor Simpson <ltaylorsimpson@gmail.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>