build: Sketch a Clang/Linux platform config.

 - Currently just tries to build a full library for i386/x86_64.

 - This is made substantially more complicated by the lack of a generalized way
   to check for/invoke cross compilers. For now, we just try and make it work
   for the matched arch, and rely on the host CC being Clang.

llvm-svn: 145322