selftests/nolibc: on x86, support exiting with isa-debug-exit

selftests/nolibc: on x86, support exiting with isa-debug-exit

QEMU, when started with "-device isa-debug-exit -no-reboot" will exit
with status code 2N+1 when N is written to 0x501. This is particularly
convenient for automated tests but this is not portable. As such we
only enable this on x86_64 when pid==1. In addition, this requires an
ioperm() call but in order not to have to define arch-specific syscalls
we just perform the syscall by hand there.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>