target/i386: avoid calling gen_eob_syscall before tb_stop

syscall and sysret only have one exit, so they do not need to
generate the end-of-translation code inline.  It can be
deferred to tb_stop.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>