[dsymutil] Require darwin in fat binary test

dsymutil uses lipo(1) to build the fat binary, which it invokes as a
process. For that, we need to only run this test on darwin systems.

Should fix: http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-ubuntu/builds/181/steps/test-check-all/logs/stdio

llvm/test/tools/dsymutil/X86/remarks-linking-fat-bundle.test

@@ -1,3 +1,5 @@
+REQUIRES : system-darwin
+
 RUN: rm -rf %t
 RUN: mkdir -p %t
 RUN: cat %p/../Inputs/remarks/fat.macho.remarks.x86 > %t/fat.macho.remarks.x86