Refactors the toolchain definitions by adding an include file to help defining GCC toolchains. This eliminates most of the duplication on Linux and Android. Adds make_global_settings for most Linux and Mac builds that should match the GYP build (this will require some more passes to get every place to be an exact match).
Fixes Android build issues in WebKit. Fixes the definition of is_android.
Renames the Linux toolchains to use the GN name of the CPU architecture so one can refer to them without conditionals. Remove conditionals from BUILDCONFIG.gn
Adds the gyp_header variable to the help (it wasn't getting inserted into the list of available variables).
R=thakis@chromium.org
Review URL: https://codereview.chromium.org/117863003
git-svn-id: http://src.chromium.org/svn/trunk/src/build@242674 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
brettw@chromium.org