target-microblaze: Cleanup dec_mul

Use tcg_gen_mul_tl for muli and mul instructions.
Use tcg_gen_muls2_tl for mulh instruction.
Use tcg_gen_mulu2_tl for mulhu instruction.
Use tcg_gen_mulsu2_tl for mulhsu instruction.

Note that this last fixes a bug, in that mulhsu was
previously treating both operands as signed, instead
of treating rb as unsigned.

Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1475011433-24456-3-git-send-email-rth@twiddle.net>