x86, fixmap: prepare fixmap_64.h for unification

Impact: cleanup

Just prepare fixmap for later mechanic unification.
No real modification on code.

  text    data     bss     dec     hex filename
4312362  527192  421924 5261478  5048a6 vmlinux-64.after
4312362  527192  421924 5261478  5048a6 vmlinux-64.before

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>