[libc++] Remove special handling of the native C++ library in benchmarks (#98529)
There were some ad-hoc settings that allowed running the benchmarks
against the native C++ Standard Library. While this ability is very
useful, it was done before the test suite was quite independent of
libc++ itself. Instead, it is better to streamline running the
benchmarks on the native standard library by using a custom Lit
configuration like we do with the test suite.
A follow-up patch will rework the integration of benchmarks with the Lit
configuration used for the test suite so that we can reuse the same
mechanism for both, making it easy to benchmark the native standard
library.
It will also make benchmarks way more user-friendly to run since we will
be able to run them like we run individual tests, which is a pain point
right now.