Fix pthread compilation flags

According to the sem_init(3) manpage, we must pass the -pthread flag at
link-time when using it.  When using pthread_join(3), this flag must also be
passed at compile time.

The flags were wrong for libremote-processor and were missing for
test-platform_host.

Change-Id: Iec2797592d6d25297c87ceb2983e87e75624a576
Signed-off-by: David Wagner <david.wagner@intel.com>