[runtimes] Fix crosscompiling after a7cad668 (D97451)

It moved the logic for CMake target arguments into llvm_ExternalProject_Add().
No handling was added for CMAKE_CROSSCOMPILING, which has a separate set of compiler_args.
This broke crosscompiling, as now the runtimes builds defaulted to the compiler's default.

I've also added passing of CMAKE_ASM_COMPILER, which was missing before although we were passing the triple for it.

Reviewed By: zero9178

Differential Revision: https://reviews.llvm.org/D97855