[libcxx] [test] Query the target platform, not the host one

target_info is inferred to WindowsLocalTI on Windows hosts unless
specified otherwise. In the latter case, it doesn't make sense to use
Windows-specific settings if the target is not Windows.

This change should not break anything, because target_info is inferred
based on what platform.system() returns. self.is_windows was set based
on the same platform.system() call.

Thanks to Sergej Jaskiewicz for the patch.

Differential Revision: https://reviews.llvm.org/D68275