Create a libc++ package

[springmeyer]

Currently the only way to get libc++ is to install the entire llvm package. To make libc++ tenable to install in CI systems quickly we should add a new llvm subpackage to package libc++.

A followup after this is done:

• [ ] Change tippecanoe package (which is statically linking libc++) to just use the clang++ and libc++ packages (and not the entire llvm package), so save on download bandwidth.

Similar to #561