[openmp, simt] Disable SIMT for user-defined reduction

The test-case included in this patch contains this target region:
...
  for (int i0 = 0 ; i0 < N0 ; i0++ )
    counter_N0.i += 1;
...

When running with nvptx accelerator, the counter variable is expected to
be N0 after the region, but instead is N0 / 32.  The problem is that rather
than getting the result for all warp lanes, we get it for just one lane.

This is caused by the implementation of SIMT being incomplete.  It handles
regular reductions, but appearantly not user-defined reductions.

For now, handle this by disabling SIMT in this case, specifically by setting
sctx->max_vf to 1.

Tested libgomp on x86_64-linux with nvptx accelerator.

gcc/ChangeLog:

2021-05-03  Tom de Vries  <tdevries@suse.de>

	PR target/100321
	* omp-low.c (lower_rec_input_clauses): Disable SIMT for user-defined
	reduction.

libgomp/ChangeLog:

2021-05-03  Tom de Vries  <tdevries@suse.de>

	PR target/100321
	* testsuite/libgomp.c/target-44.c: New test.