[nvptx, libgomp] Don't launch with num_workers == 0

When using a compiler build with:
...
+#define PTX_DEFAULT_VECTOR_LENGTH PTX_CTA_SIZE
+#define PTX_MAX_VECTOR_LENGTH PTX_CTA_SIZE
...
and running the libgomp testsuite, we run into an execution failure in
parallel-loop-1.c, due to a cuda launch failure:
...
  nvptx_exec: kernel f6_none_none$_omp_fn$0: launch gangs=480, workers=0, \
    vectors=1024

libgomp: cuLaunchKernel error: invalid argument
...
because workers == 0.

The workers variable is set to 0 here in nvptx_exec:
...
                workers = blocks / actual_vectors;
...
because actual_vectors is 1024, and blocks is 768:
...
cuOccupancyMaxPotentialBlockSize: grid = 10, block = 768
...

Fix this by ensuring that workers is at least one.

2019-01-09  Tom de Vries  <tdevries@suse.de>

	* plugin/plugin-nvptx.c (nvptx_exec): Make sure to launch with at least
	one worker.

From-SVN: r267746