[gn] Don't pass -fprofile-instr-generate to linker on Windows

Avoids a warning from the linker. The user still has to put the resource
directory on the linker search path, and I can't find a clean way to do
that automatically in gn.

llvm/utils/gn/build/BUILD.gn

@@ -233,7 +233,9 @@ config("compiler_defaults") {
       "-fprofile-instr-generate=" +
           rebase_path("$root_build_dir/profiles/%4m.profraw"),
     ]
-    ldflags += [ "-fprofile-instr-generate" ]
+    if (host_os != "win") {
+      ldflags += [ "-fprofile-instr-generate" ]
+    }
   }
 
   # Deterministic build setup, see