aarch64: fix cmpbr-3.c tests

The assembly tests in `cmpbr-3.c` were failing when run with an old
version of `gas` which did not recognise the extension. Fix by changing
`dg-do assemble` to `dg-do compile`.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/cmpbr-3.c: `dg-do assemble` => `dg-do compile`.

gcc/testsuite/gcc.target/aarch64/cmpbr-3.c

@@ -1,4 +1,4 @@
-/* { dg-do assemble } */
+/* { dg-do compile } */
 /* { dg-options "-O2" } */
 
 #pragma GCC target "+cmpbr"