lib: move muldiv64()

lib: move muldiv64()

Make this a separate archive member under lib/. While doing so, don't
move latently broken x86 assembly though: Fix the constraints, such
that properly extending inputs to 64-bit won't just be a side effect of
needing to copy registers, and such that we won't fail to clobber %rdx.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>