gn build: Create a template for unix toolchains.

gn build: Create a template for unix toolchains.

Also change the toolchain description to use current_os instead of
host_os so that the template can be used for cross builds, and add
a current_os to the win toolchain to match the unix toolchain.

Differential Revision: https://reviews.llvm.org/D56576

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350977 91177308-0d34-0410-b5e6-96231b3b80d8