target-i386: Create gen_lea_v_seg

Add forgotten zero-extension in the TARGET_X86_64, !CODE64, ss32 case;
use this new function to implement gen_string_movl_A0_EDI,
gen_string_movl_A0_ESI, gen_add_A0_ds_seg.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1450379966-28198-2-git-send-email-rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>