[libc++][tests] Use CMake provided paths for includes and libdir instead of hardcoding them

In the new-style testing configurations, we were hardcoding paths to the
`include` and `lib` directories, which was incorrect but always went
unnoticed because the hardcoded values always happened to match the
actual value.

When using new-style configs with the bootstrapping build, this falls
appart -- and we never noticed this because the bootstrapping build was
still using old style configs.

This patch removes the %{install} substitution, which makes it too
tempting to hardcode installation paths, and it also switches the
bootstrapping build to actually using new-style configs like we
always intended to do.

Differential Revision: https://reviews.llvm.org/D121700