Add a multithreaded test case.

gcc/testsuite/
	* jit.dg/test-threads.c: New test case, running all of the
	individual test cases in separate threads.
	* jit.dg/test-combination.c: Move inclusion of the various
	individual testcases into...
	* jit.dg/all-non-failing-tests.h: ...this new file, and rename
	TEST_COMBINATION to COMBINED_TEST.
	* jit.dg/harness.h: Respond to new macro MAKE_DEJAGNU_H_THREADSAFE
	by hacking up <dejagnu.h> to be threadsafe.  Rename
	TEST_COMBINATION to COMBINED_TEST.
	* jit.dg/jit.exp (proc jit-dg-test): Add "-lpthread" when building
	test-threads.exe.