Add new tasks for the "Linux" platform. These run on the same docker image as
the Linux64 builds, but that image has been modified to contain a bunch of
*.i686 packages required to cross-compile for i686. Due to yum's propensity
for resolving dependencies without regard to architecture, with this patch the
system-setup.sh script lists both architectures of each file explicitly.
This also leaves `gcc` installed for user convenience in installing Python
extensions, NPM modules, etc.
This also includes 'subversion' for clang builds (bug 1208029)
--HG--
extra : commitid : GfCTCchyHo6
extra : rebase_source : 8b15da0ed7adefa084b7195a98f63f73564a3d94
Introduces a centos6-builder image and refactors desktop-build to use it.
(64-bit only)
--HG--
extra : commitid : BMO91NqNEEh
extra : rebase_source : 85a32f4f88f7d52b86b25516022c44e78b110fd2
