[CMake] Fix runtimes build for host Windows (default target)

[CMake] Fix runtimes build for host Windows (default target)

When building runtimes, the compiler name (e.g. clang, clang-cl) is set based on
`CMAKE_SYSTEM_NAME` passed to `llvm_ExternalProject_Add()` through `CMAKE_ARGS` argument.
This mechanism doesn't work well if the target is Windows host.
`runtime_default_target()`/`builtin_default_target()` doesn't provide a way
to specify `CMAKE_SYSTEM_NAME` and doesn't set it either.

This patch appends variables specified in `RUNTIMES_CMAKE_ARGS`/`BUILTINS_CMAKE_ARGS`
to `CMAKE_ARGS` argument of `llvm_ExternalProject_Add()` in the case of called
from `runtime_default_target()`/`builtin_default_target()` thus in particular
it allows passing CMAKE_SYSTEM_NAME whenever it is required.

Reviewed By: phosek, compnerd, plotfi

Differential Revision: https://reviews.llvm.org/D81877