[CMake] Set ASM compiler for external projects

This is necessary on Windows, otherwise CMake fails. It's not
conventional on Windows to use cl for assembly (you'd use ml or ml64
instead), but CMake has a separate ASM_MASM mode for that, and clang-cl
works fine for assembly so we'll use that on Windows for consistency.

Differential Revision: https://reviews.llvm.org/D73522