exec: make mmap_lock/mmap_unlock globally available

There is some iffy lock hierarchy going on in translate-all.c.  To
fix it, we need to take the mmap_lock in cpu-exec.c.  Make the
functions globally available.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>