GN: mark newlib_pnacl toolchain as clang

newlib_pnacl toolchain is based on clang so it should be marked as such
to use the correct cflags.

BUG= https://github.com/domokit/mojo/issues/396
R=smklein@chromium.org

Review URL: https://codereview.chromium.org/1411173004 .

build/toolchain/nacl/BUILD.gn

diff --git a/build/toolchain/nacl/BUILD.gn b/build/toolchain/nacl/BUILD.gn
index 5329f18..2c641b1 100644
--- a/build/toolchain/nacl/BUILD.gn
+++ b/build/toolchain/nacl/BUILD.gn
@@ -30,6 +30,7 @@
       rebase_path("${nacl_toolchain_dir}/${toolchain_package}/bin/pnacl-",
                   root_build_dir)
 
+  is_clang = true
   cc = toolprefix + "clang"
   cxx = toolprefix + "clang++"
   ar = toolprefix + "ar"