arm,testsuite: Add -mtune=cortex-m55 to dlstp-int8x16.c

Like dlstp-compile-asm-1.c, this test would fail if GCC is configured
with non-default options, such as -mtune=cortex-a9.

Force -mtune=cortex-m55 to avoid this unexpected issue.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/mve/dlstp-int8x16.c: Add -mtune=cortex-m55

gcc/testsuite/gcc.target/arm/mve/dlstp-int8x16.c

@@ -1,6 +1,6 @@
 /* { dg-do compile { target { arm*-*-* } } } */
 /* { dg-require-effective-target arm_v8_1m_mve_ok } */
-/* { dg-options "-O2 -save-temps" } */
+/* { dg-options "-O2 -save-temps -mtune=cortex-m55" } */
 /* { dg-add-options arm_v8_1m_mve } */
 
 #include <arm_mve.h>