x86, fixmap: define FIXADDR_BOOT_* and redefine FIX_ADDR_SIZE

Impact: new interface, not yet used

Now, with these macros, x86_64 code can know where start the
permanent and non-permanent fixed mapped address.
This patch make these macros equal fixmap_32.h for future
x86 integration.

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>