gn build: Create a variable for the host toolchain and start using it in the tblgen template.

Differential Revision: https://reviews.llvm.org/D56575

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350964 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed