tests: compile tst-mmap with "-z now"

Compile tst-mmap.so with "-z now", so that symbols are resolved at load
time instead of when needed.

The problem was that tst-mmap uses wait_until, which runs the functions
sched::thread::wait() and sched::thread::stop_wait() while preemption is
disabled. In about 1 in 10 runs of tst-mmap.so, this caused an assertion-
failure crash (symbol resolution caused sleeping while preemption is disabled).

Fixes #256.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>