tcg: Avoid recursion in tcg_gen_mulu2_i32

tcg: Avoid recursion in tcg_gen_mulu2_i32

We have a test for one of TCG_TARGET_HAS_mulu2_i32 or
TCG_TARGET_HAS_muluh_i32 being defined, but the test
became non-functional when we changed to always define
all of these macros.

Replace this with a build-time test in tcg_gen_mulu2_i32.

Fixes: 25c4d9cc845 ("tcg: Always define all of the TCGOpcode enum members.")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1435
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>