Revert "testsuite: Disable strub on AIX."

This reverts commit 9773ca5198.

The fix was in commit cb62101787.

The changes to strub-unsupported* were incorrect, those tests verify
the error messages issued when strub support is properly disabled with
TARGET_HAVE_STRUB_SUPPORT_FOR.

gcc/testsuite/c-c++-common/strub-unsupported-2.c

@@ -1,5 +1,4 @@
 /* { dg-do compile } */
-/* { dg-require-effective-target strub } */
 
 /* Check that, when strub is not supported (so no dg-required-effective-target
    strub above), we report when pointers to strub functions are called.  This

gcc/testsuite/c-c++-common/strub-unsupported-3.c

@@ -1,5 +1,4 @@
 /* { dg-do compile } */
-/* { dg-require-effective-target strub } */
 
 /* Check that, when strub is not supported (so no dg-required-effective-target
    strub above), we report when strub functions that are not defined are

gcc/testsuite/c-c++-common/strub-unsupported.c

@@ -1,5 +1,4 @@
 /* { dg-do compile } */
-/* { dg-require-effective-target strub } */
 
 /* Check that, when strub is not supported (so no dg-required-effective-target
    strub above), we report when strub functions are defined, and when they're

gcc/testsuite/lib/target-supports.exp

@@ -1304,10 +1304,6 @@ proc check_stack_check_available { stack_kind } {
 
 # Return 1 if the target supports stack scrubbing.
 proc check_effective_target_strub {} {
-    # strub is not supported on AIX.
-    if { [istarget powerpc*-*-aix*] } {
-	return 0
-    }
     return [check_no_compiler_messages strub assembly {
 	void __attribute__ ((__strub__)) fn (void) {}
     } ""]