[clang] XFAIL the Xclangas.s test on AIX.

Clang on AIX does not use the integrated assembler.

https://lab.llvm.org/buildbot/#/builders/64/builds/3074

Author: alexrp

clang/test/Driver/Xclangas.s

@@ -2,3 +2,4 @@
 // RUN: %clang -### -Werror -Xclangas -target-feature -Xclangas=+v5t %s 2>&1 | FileCheck %s
 // CHECK: -cc1as
 // CHECK-SAME: "-target-feature" "+v5t"
+// XFAIL: target={{.*}}-aix{{.*}}