Adjust 'libgomp.c/max_vf-*.c'

Adjust 'libgomp.c/max_vf-*.c'

For configurations where both GCN and nvptx offloading are enabled, we get:

    PASS: libgomp.c/max_vf-1.c (test for excess errors)
    PASS: libgomp.c/max_vf-1.c scan-tree-dump-times ompexp "GOMP_MAX_VF" 2
    PASS: libgomp.c/max_vf-1.c scan-tree-dump-times ompexp "__builtin_GOMP_parallel_loop_nonmonotonic_dynamic \\(.*, D\\.[0-9]*, 0\\);" 1
    PASS: libgomp.c/max_vf-1.c scan-amdgcn-amdhsa-offload-tree-dump-times optimized "__builtin_GOMP_parallel_loop_nonmonotonic_dynamic \\(.*, 64, 0\\);" 1
    FAIL: libgomp.c/max_vf-1.c scan-nvptx-none-offload-tree-dump-times optimized "__builtin_GOMP_parallel_loop_nonmonotonic_dynamic \\(.*, 64, 0\\);" 1
    FAIL: libgomp.c/max_vf-1.c scan-amdgcn-amdhsa-offload-tree-dump-times optimized "__builtin_GOMP_parallel_loop_nonmonotonic_dynamic \\(.*, 7, 0\\);" 1
    PASS: libgomp.c/max_vf-1.c scan-nvptx-none-offload-tree-dump-times optimized "__builtin_GOMP_parallel_loop_nonmonotonic_dynamic \\(.*, 7, 0\\);" 1

Avoid these FAILs via 'only_for_offload_target [...]'.  Also, for consistency
with other libgomp test cases, use effective-target specifiers of the libgomp
test suite.  Fix-up for recent commit d334f729e53867b838e867375b3f475ba793d96e
"openmp: Add testcases for omp_max_vf".

	libgomp/
	* testsuite/libgomp.c/max_vf-1.c: Adjust.
	* testsuite/libgomp.c/max_vf-2.c: Likewise.