target/i386: optimize cross-page direct jumps in softmmu

Instead of unconditionally exiting to the exec loop, use the
gen_jr helper to jump to the target if it is valid.

Perf impact: see next commit's log.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1493263764-18657-10-git-send-email-cota@braap.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
