[GCN] Fix run-time variable 'num_workers'

[GCN] Fix run-time variable 'num_workers'

... which currently has *not* been forced to 'num_workers (1)'.

In addition to the testcases modified here, this also fixes:

    FAIL: libgomp.oacc-c/../libgomp.oacc-c-c++-common/mode-transitions.c -DACC_DEVICE_TYPE_radeon=1 -DACC_MEM_SHARED=0 -foffload=amdgcn-amdhsa  -O0  execution test
    [Etc.]

    mode-transitions.exe: [...]/libgomp.oacc-c-c++-common/mode-transitions.c:702: t17: Assertion `arr_b[i] == (i ^ 31) * 8' failed.

	libgomp/
	* plugin/plugin-gcn.c (gcn_exec): Force 'num_workers (1)'
	unconditionally.
	* testsuite/libgomp.oacc-c-c++-common/acc_prof-kernels-1.c:
	Update.
	* testsuite/libgomp.oacc-c-c++-common/parallel-dims.c: Likewise.
	* testsuite/libgomp.oacc-c-c++-common/routine-wv-2.c: Likewise.