tcg-s390: Use qemu_getauxval in query_facilities

No need to set up a SIGILL signal handler for detection anymore.

Remove a ton of sanity checks that must be true, given that we're
requiring a 64-bit build (the note about 31-bit KVM is satisfied
by configuring with TCI).

Signed-off-by: Richard Henderson <rth@twiddle.net>
