The kernel now has asm/cpu-features.h: use those macros instead of
inventing our own.

Also spell out definition of CR0_RESEVED_BITS (no code change) and fix typo.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Avi Kivity <avi@qumranet.com>