test: disable cross process sem tests on non-Linux

test: disable cross process sem tests on non-Linux

How to make this portable:

- MAP_ANONYMOUS -> MAP_ANON (OSX)

- sem_init (anonymous semaphore) needs to be replaced by named
semaphores using sem_open/sem_close. Use a memory address of the sem_t
variable to hack anonymous semaphore behavior.

- sem_getvalue isn't supported on OSX. it is used here to do
sem_wait/sem_post to bring a semaphore back to a specific value. to get
around this we may need to restructure the test so that the semaphore
can be destroyed and re-initialized rather than inspected as its
currently being done.

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>