[libc++] Simplify how with_system_cxx_lib and availability features are defined

[libc++] Simplify how with_system_cxx_lib and availability features are defined

Instead of spamming a bunch of available features that are not actually
used anywhere, only set those that are actually used in the test suite.
In the future, this should probably be based on the target triple only,
with the ability to have wildcards in the triple.