Fix test failure for targets with varying uwtable defaults

[chill]

Summary

Depending on toolchain and ABI, a target might not output DWARF unwind tables by default.
Run the test for a target with a known behaviour, test coverage is not reduced.

[dmgreen]

I think if you are just fixing buildbots, this kind of thing doesn't need review. LGTM is either case :)

clang/test/CodeGen/uwtable-attr.c

 // Test that function and modules attributes react on the command-line options,
 // it does not state the current behaviour makes sense in all cases (it does not).
 
-// RUN: %clang -S -emit-llvm -o - %s                                                    | FileCheck %s -check-prefixes=CHECK,DEFAULT
-// RUN: %clang -S -emit-llvm -o - %s -funwind-tables    -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,TABLES
-// RUN: %clang -S -emit-llvm -o - %s -fno-unwind-tables -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,NO_TABLES
-
-// RUN: %clang -S -emit-llvm -o - -x c++ %s                                                                     | FileCheck %s -check-prefixes=CHECK,DEFAULT
-// RUN: %clang -S -emit-llvm -o - -x c++ %s                  -funwind-tables    -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,TABLES
-// RUN: %clang -S -emit-llvm -o - -x c++ %s  -fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,NO_TABLES
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - %s                                                    | FileCheck %s -check-prefixes=CHECK,DEFAULT
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - %s -funwind-tables    -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,TABLES
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - %s -fno-unwind-tables -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,NO_TABLES
+
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - -x c++ %s                                                                     | FileCheck %s -check-prefixes=CHECK,DEFAULT
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - -x c++ %s                  -funwind-tables    -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,TABLES
+// RUN: %clang -target x86_64-linux -S -emit-llvm -o - -x c++ %s  -fno-exceptions -fno-unwind-tables -fno-asynchronous-unwind-tables | FileCheck %s -check-prefixes=CHECK,NO_TABLES
+
+// REQUIRES: x86-registered-target
 
 #ifdef __cplusplus
 extern "C" void g(void);
 struct S { ~S(); };
 extern "C" int f() { S s; g(); return 0;};
 #else
 void g(void);
 int f() { g(); return 0; };