Implement a helper library for POSIX semantics syscalls.

Since Linux kernel supported threads are not POSIX threads
and the glibc pthread library only supports POSIX semantics
for 9 system calls, to fully support the POSIX semantics for
a process sharing its security state across all of its
threads, we've created libpsx.

This commit also includes a threading test in tests/ for
this new psx_syscall() abstraction - one that transparently
mirrors calling POSIX-needing semantics syscalls over
all running threads.

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>
diff --git a/Makefile b/Makefile
index be8f38e..08d2570 100644
--- a/Makefile
+++ b/Makefile
@@ -16,6 +16,7 @@
 ifeq ($(GOLANG),yes)
 	$(MAKE) -C go $@
 endif
+	$(MAKE) -C tests $@
 	$(MAKE) -C progs $@
 	$(MAKE) -C doc $@
 	$(MAKE) -C kdebug $@