[CMake] Make sure _cmake_system_name has a default

We currently try to pick it up from the CMake arguments passed to llvm_ExternalProject_Add but
if there isn't an explicit option passed, we should reflect CMake's own default behaviour
of targeting the host, since we'll make decisions about what tools to use for the build based on
the setting. Otherwise, we'll get different behaviour between configuring an external project with
the default target and configuring with an explicit one targeting the same platform.

Reviewed By: stevewan, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D88157

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