tcg/{i386, s390x}: Add earlyclobber to the op_add2's first output

i386 and s390x implementations of op_add2 require an earlyclobber,
which is currently missing. This breaks VCKSM in s390x guests. E.g., on
x86_64 the following op:

    add2_i32 tmp2,tmp3,tmp2,tmp3,tmp3,tmp2   dead: 0 2 3 4 5  pref=none,0xffff

is translated to:

    addl     %ebx, %r12d
    adcl     %r12d, %ebx

Introduce a new C_N1_O1_I4 constraint, and make sure that earlyclobber
of aliased outputs is honored.

Cc: qemu-stable@nongnu.org
Fixes: 82790a87 ("tcg: Add markup for output requires new register")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230719221310.1968845-7-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>