middle-end: force AMDGCN test for vect-early-break_18.c to consistent architecture [PR119286]

The given test is intended to test vectorization of a strided access done by
having a step of > 1.

GCN target doesn't support load lanes, so the testcase is expected to fail,
other targets create a permuted load here which we then then reject.

However some GCN arch don't seem to support the permuted loads either, so the
vectorizer tries a gather/scatter.  But the indices aren't supported by some
target, so instead the vectorizer scalarizes the loads.

I can't really test for which architecture is being used by the compiler, so
instead this updates the testcase to use one single architecture so we get a
consistent result.

gcc/testsuite/ChangeLog:

	PR target/119286
	* gcc.dg/vect/vect-early-break_18.c: Force -march=gfx908 for amdgcn.

gcc/testsuite/gcc.dg/vect/vect-early-break_18.c

@@ -2,7 +2,7 @@
 /* { dg-do compile } */
 /* { dg-require-effective-target vect_early_break } */
 /* { dg-require-effective-target vect_int } */
-
+/* { dg-additional-options "-march=gfx908" { target amdgcn*-*-* } } */
 /* { dg-additional-options "-Ofast" } */
 
 /* { dg-final { scan-tree-dump "LOOP VECTORIZED" "vect" { target vect_load_lanes } } } */