XFAIL a test on ppc64

Buildbot failure:
https://lab.llvm.org/buildbot/#/builders/105/builds/13141
which provides no details about why it failed, but the only failure
reports are for ppc64 bots.

Author: pogo59

clang/test/Driver/as-no-warnings.c

@@ -14,6 +14,7 @@
 // REQUIRES: clang-driver
 // REQUIRES: x86-registered-target
 // REQUIRES: system-linux
+// XFAIL: ppc64
 
 // CHECK: "-cc1" {{.*}} "-massembler-no-warn"
 // CHECK-NOIAS: "--no-warn"