x86, fixmap: unify fixmap.h

Impact: unification

This patch unify fixmap_32.h and fixmap_64.h into fixmap.h.
Things that we can't merge now are using CONFIG_X86_{32,64}
(e.g.:vsyscall and EFI)

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>