libgomp: Fix default value of GOMP_SPINCOUNT [PR 109062]

When OMP_WAIT_POLICY is not specified, current implementation will cause
icv flag GOMP_ICV_WAIT_POLICY unset, so global variable wait_policy
will remain its uninitialized value. Initialize it to -1 to make
GOMP_SPINCOUNT behavior consistent with its description.

libgomp/ChangeLog:

	PR libgomp/109062
	* env.c (wait_policy): Initialize to -1.
	(initialize_icvs): Initialize icvs->wait_policy to -1.
	* testsuite/libgomp.c-c++-common/pr109062.c: New test.