tcg-sparc: Implement muls2_i32

Using the 32-bit SMUL is a tad more efficient than
resorting to extending and using the 64-bit MULX.

Signed-off-by: Richard Henderson <rth@twiddle.net>
tcg-sparc: Support addsub2_i64

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