hw/apic.c: use __builtin funtions instead of assembly code

Suggested by malc.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5465 c046a42c-6fe2-441c-8c8c-71466251a162