[libc++] List the set of __config_site macros used for features

Instead of creating Lit features for all __config_site macros automatically,
only do so for macros that generate features actually used in the test
suite. This makes it easier to know which ones are supported by the test
suite at a glance.

Note that the `libcpp-abi-version-vN` is dropped altogether, but it
wasn't used anywhere.