[CMake] Passthrough CMAKE_SYSTEM_NAME to default builtin and runtimes target

When building the default builtin and runtimes target, set the
CMAKE_SYSTEM_NAME to the current one. This is not necessary on
Linux and Darwin, but it appears to be necessary on Windows,
otherwise CMake fails.

Differential Revision: https://reviews.llvm.org/D73811

GitOrigin-RevId: 491a4a7ac9e39fd4cc20a21e7da1961dfc7fc275
1 file changed