amdgcn: remove unnecessary scalar cache flush

The exit code isn't actually written via the scalar cache so the cache flush
is not actually needed.

diff --git a/newlib/libc/machine/amdgcn/exit-value.h b/newlib/libc/machine/amdgcn/exit-value.h
index 7aa2508bb..6b9d2411b 100644
--- a/newlib/libc/machine/amdgcn/exit-value.h
+++ b/newlib/libc/machine/amdgcn/exit-value.h
@@ -32,7 +32,6 @@ exit_with_int (int val)
   *return_value = val;
 
   /* Terminate the current kernel.  */
-  asm ("s_dcache_wb");
   asm ("s_endpgm");
   __builtin_unreachable ();
 }