gcc.dg/independent-cloneids-1.c: Skip for mmix.

Regular ELF label definitions for this test-case, matched by the
regexps, e.g.:
 /* { dg-final { scan-assembler-times {(?n)^_*bar[.$_]constprop[.$_]0:} 1 } } */
typically look like this:
bar_constprop.0:

For MMIX, they look like this:
bar_constprop::0	IS @

I think it's better to just skip the test for MMIX than further
uglifying the matching regexps, since the test is IIUC general
enough that nothing in the target port can reasonably make a
difference: it passes for all targets or fail for all targets.

gcc/testsuite:
	* gcc.dg/independent-cloneids-1.c: Skip for mmix.
	flag_stack_usage_info.

gcc/testsuite/gcc.dg/independent-cloneids-1.c

@@ -1,5 +1,6 @@
 /* { dg-do compile } */
 /* { dg-options "-O3 -fipa-cp -fipa-cp-clone"  } */
+/* { dg-skip-if "Odd label definition syntax" { mmix-*-* } } */
 
 extern int printf (const char *, ...);