[libgomp, testsuite, openacc] Don't use const int for dimensions

Const int is handled differently at -O0 for -xc and -xc++, which can cause noise
in testsuite/libgomp.oacc-c-c++-common test-cases (which are both run for c and
c++) if const int is used for launch dimensions.

Fix this by using #defines instead.

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

	PR target/88756
	* testsuite/libgomp.oacc-c-c++-common/reduction-1.c (ng, nw, vl): Use
	#define instead of "const int".
	* testsuite/libgomp.oacc-c-c++-common/reduction-2.c (ng, nw, vl): Same.
	* testsuite/libgomp.oacc-c-c++-common/reduction-3.c (ng, nw, vl): Same.
	* testsuite/libgomp.oacc-c-c++-common/reduction-4.c (ng, nw, vl): Same.
	* testsuite/libgomp.oacc-c-c++-common/reduction-5.c (ng, nw, vl): Same.

From-SVN: r267747