[libcxx] [test] Move the is_<platform> functions down to subclasses

If cross testing (and manually specifying a LIBCXX_TARGET_INFO in the
cmake configuration, as the default is to match the build platform),
we want the accessors for querying the target platform, is_windows,
is_darwin, to return the right value depending on which target info
class is used, not based on what platform is running the build and
driving the tests.

When LIBCXX_TARGET_INFO isn't defined, the right target info class
is chosen automatically based on the platform one is running on, so
this shouldn't make any practical difference for such setups.

Differential Revision: https://reviews.llvm.org/D98045