tcg: Implement muls2 with mulu2

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-arm: Implement muls2_i32

We even had the encoding of smull already handy...

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg: Apply life analysis to 64-bit multiword arithmetic ops

Signed-off-by: Richard Henderson <rth@twiddle.net>
Tidy i386 muls2 use