Avoid clobbering %ebx/%rbx in inline assembly with Apple's CC
* efiemu/runtime/efiemu.c (write_cmos): use %cl instead of %bl as
temporary storage
* include/grub/i386/tsc.h (grub_get_tsc): restore %rbx/%ebx when
using Apple's CC
(grub_cpu_is_tsc_supported): likewise
* loader/i386/xnu.c (guessfsb): restore %rbx/%ebx in inline assembly