[libc++] Switch to the new testing configurations by default

We've been meaning to remove support for the legacy testing configuration
for a long time. This patch switches the default from the legacy config
to the appropriate new-style configuration based on a few hints.

We've been running with the new-style configuration for more than a year
in our CI, however it's possible that this will uncover issues with some
users that run the tests on platforms that we don't support yet with the
new-style configs. Unfortunately, there is no way to know about it other
than to land this patch and see whether anything breaks.

Differential Revision: https://reviews.llvm.org/D121632