This is a logical reversion of 2ad0450054, though
there have been additions to the set of mmu indexes
since then. The impetus to that original patch,
"9-15 will use shorter assembler instructions when
run on a x86-64 host" is now handled generically.
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>