x86, fixmap: rename __FIXADDR_SIZE and __FIXADDR_BOOT_SIZE

Impact: rename

Rename __FIXADDR_SIZE to FIXADDR_SIZE
and __FIXADDR_BOOT_SIZE to FIXADDR_BOOT_SIZE.

Signed-off-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Acked-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>