Fix 'libgomp.c/simd-math-1.c' configuration

Fix 'libgomp.c/simd-math-1.c' configuration

If nvptx offloading is configured in addition to GCN, we see:

    FAIL: libgomp.c/simd-math-1.c (test for excess errors)
    UNRESOLVED: libgomp.c/simd-math-1.c compilation failed to produce executable

    x86_64-pc-linux-gnu-accel-nvptx-none-gcc: error: unrecognized command-line option '-mstack-size=3000000'

Thus, restrict that ooption to GCN offloading compilation, and on the other
hand, there's no reason to skip this test for non-GCN offloading execution:
even if not SIMD-vectorized there, we still benefit from correctness testing.

	libgomp/
	* testsuite/libgomp.c/simd-math-1.c: Fix configuration.