[doc] Use cmake's -S option to simplify the build instructions

Now that llvm's minimum version is 3.13.4 anyway and `-S` is
introduced in the same version[1], let's use this little
time saving option to simplify the build instructions.

[1]: https://cgold.readthedocs.io/en/latest/glossary/-S.html



Authored-By: Ebrahim Byagowi <ebrahim@gnu.org>

Differential-Revision: https://reviews.llvm.org/D95049