target-i386: Rewrite gen_enter inline

Use gen_lea_v_seg for centralized segment base knowledge.  Unify
code across 32- and 64-bit.  Fix note about "must save state"
before using the out-of-line helpers.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1450379966-28198-8-git-send-email-rth@twiddle.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>