host-utils: Use __int128_t for mul[us]64

Replace some x86_64 specific inline assembly with something that
all 64-bit hosts ought to optimize well.  At worst this becomes
a call to the gcc __multi3 routine, which is no worse than our
implementation in util/host-utils.c.

With gcc 4.7, we get identical code generation for x86_64.  We
now get native multiplication on ia64 and s390x hosts.  With minor
improvements to gcc we can get it for ppc64 as well.

Signed-off-by: Richard Henderson <rth@twiddle.net>
host-utils: Improve mulu64 and muls64

The new formulation makes better use of add-with-carry type insns
that the host may have.  Use gcc's sign adjustment trick to avoid
having to perform a 128-bit negation.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tests: Add unit tests for mulu64 and muls64

Signed-off-by: Richard Henderson <rth@twiddle.net>