Tweak error return value for acc_set_cuda_stream.

Tweak error return value for acc_set_cuda_stream.

The return value of acc_set_cuda_stream is unspecified in OpenACC 2.6.
The testsuite changes might be unnecessary with the current async code.

	libgomp/
	* oacc-cuda.c (acc_set_cuda_stream): Return 0 on error/invalid
	arguments.
	* testsuite/libgomp.oacc-c-c++-common/lib-84.c: Handle unnumbered
	async stream being an alias for a numbered async stream.
	* testsuite/libgomp.oacc-c-c++-common/lib-85.c: Likewise.

(cherry picked from openacc-gcc-9-branch commit
6b0f72aad2030b3efa651652d75ad3d3cc9a76f6)