tcg: Expand fallback add2 with 32-bit operations

tcg: Expand fallback add2 with 32-bit operations

No need to expand to i64 to perform the add.
This is smaller on a loongarch64 host, e.g.

	bstrpick_d  r28, r27, 31, 0
	bstrpick_d  r29, r24, 31, 0
	add_d       r28, r28, r29
	addi_w      r29, r28, 0
	srai_d      r28, r28, 32
  ---
	add_w       r28, r27, r24
	sltu        r29, r28, r24

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>