[libc++] Build GoogleBenchmark directly from Lit (#224192)

Previously, we would build GoogleBenchmark against the just-built
library, not against the library being tested. When testing historical
versions of libc++ or other standard libraries, this breaks. So instead
of building Google Benchmark against the just-built library in CMake, do
it from Lit as part of the test suite's configuration.

I'm not a huge fan of using Lit as a poor man's build system and we
should make the CMake test suite self-contained, however this is a step
in the right direction and it removes a major coupling between the test
suite and the regular libc++ build.

GitOrigin-RevId: 1bfe9893f3dc12f1c0f9546564519f02601b2192