target-i386: Use gen_lea_v_seg in pusha/popa

More centralization of handling of segment bases.
Also fixes the note about 16-bit wrap around not fully handled.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1450379966-28198-7-git-send-email-rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>