Rework handling of os and cpu_arch in GN.

tools/gn/format_test_data/030.golden

 # Don't separate simple statements in a scope.
+
+import("//testing/test.gni")
+

[Dirk Pranke, 2015/02/11 02:34:31]

This is an unrelated fix I discovered while trying to run gn_unittests, which I
guess we're still not running on the bots (apart from the gn_upload bots).

[scottmg, 2015/02/11 03:06:20]

Weird, oops. :/

Er, no... was part of a mistaken change in a big refactor.
https://codereview.chromium.org/824263004 Yeah, we should get the tests on the
main bots.

[tfarina, 2015/02/11 13:04:48]

I looked into this and it was also disabled on gn_upload.

I made CL https://codereview.chromium.org/920493002 to address this.

 test("something") {
   if (is_linux) {
     sources -= [ "file_version_info_unittest.cc" ]
     sources += [ "nix/xdg_util_unittest.cc" ]
     defines = [ "USE_SYMBOLIZE" ]
     configs += [ "//build/config/linux:glib" ]
   }
 }