gn build: Stop passing -o to ar.

The -o flag means something different to ar than what appears to be
intended here. Also, llvm-ar doesn't accept the flag in this position.

Differential Revision: https://reviews.llvm.org/D56426

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350604 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/build/toolchain/BUILD.gn b/utils/gn/build/toolchain/BUILD.gn
index 927a53d..e1254af 100644
--- a/utils/gn/build/toolchain/BUILD.gn
+++ b/utils/gn/build/toolchain/BUILD.gn
@@ -51,7 +51,7 @@
       # Remove the output file first so that ar doesn't try to modify the
       # existing file.
       command =
-          "rm -f {{output}} && ar rcsDT {{arflags}} -o {{output}} {{inputs}}"
+          "rm -f {{output}} && ar rcsDT {{arflags}} {{output}} {{inputs}}"
     }
     description = "AR {{output}}"
     outputs = [