cmake fill `cmake_args` when cross-compiling external project with non-clang compiler

This makes it possible to crosscompile runtimes with cl.exe on Windows.

An external project is completely misconfigured otherwise because
cmake_args is set only for native builds or builds crosscompiled with
clang.

Differential Revision: https://reviews.llvm.org/D122578
Reviewed By: beanz, compnerd