Fix as-w-option.c on Windows where no assembler exists

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374936 91177308-0d34-0410-b5e6-96231b3b80d8

test/Driver/as-w-option.c

@@ -1,4 +1,4 @@
-// RUN: %clang -### %s -c -o tmp.o -fno-integrated-as -Wa,-W 2>&1 | FileCheck -check-prefix=CHECK-NOIAS %s
+// RUN: %clang -### %s -c -o tmp.o -fno-integrated-as --target=x86_64-linux-gnu -Wa,-W 2>&1 | FileCheck -check-prefix=CHECK-NOIAS %s
 // RUN: %clang -### %s -c -o tmp.o -integrated-as -Wa,-W 2>&1 | FileCheck -check-prefix=CHECK-IAS %s
 
 // CHECK-IAS: "-cc1" {{.*}} "-massembler-no-warn"