runtime/Linux: Initial support for tying compiler-rt build into Clang build on

Linux.
 - Currently just builds a full library, and only on x86, and only for the
   target arch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145672 91177308-0d34-0410-b5e6-96231b3b80d8