[RS6000] dimode_off.c test

[Alan Modra]

This tests behaviour near the limit of 16-bit signed offsets.  If
power10 prefix instructions are enabled, no such testing occurs.

	* gcc.target/powerpc/dimode_off.c: Add -mno-prefixed to options.

Regstrapped powerpc64le-linux power10 and power8.  OK?

diff --git a/gcc/testsuite/gcc.target/powerpc/dimode_off.c b/gcc/testsuite/gcc.target/powerpc/dimode_off.c
index 19ca40c508b..12718eafdd3 100644
--- a/gcc/testsuite/gcc.target/powerpc/dimode_off.c
+++ b/gcc/testsuite/gcc.target/powerpc/dimode_off.c
@@ -1,5 +1,5 @@
 /* { dg-do assemble } */
-/* { dg-options "-O2 -fno-align-functions -fno-asynchronous-unwind-tables -mtraceback=no -save-temps" } */
+/* { dg-options "-O2 -fno-align-functions -fno-asynchronous-unwind-tables -mtraceback=no -mno-prefixed -save-temps" } */
 
 void w1 (void *x, long long y) { *(long long *) (x + 32767) = y; }
 void w2 (void *x, long long y) { *(long long *) (x + 32766) = y; }

-- 
Alan Modra
Australia Development Lab, IBM