clang/scripts/build.py: Set CMAKE_INSTALL_LIBDIR for libxml2 and zstd

The rest of the script assumes the static libraries will be installed
into `${CMAKE_INSTALL_PREFIX}/lib`, but depending on the distro (e.g.
Fedora) and platform the installation directory might be lib64 instead,
as set by CMake's GNUInstallDirs.cmake module.

Be explicit and make sure the libraries always get installed into the
expected directory.

Change-Id: Id6d91c143423762094de2df7048e6238e1890af5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6110461
Reviewed-by: Hans Wennborg <hans@chromium.org>
Auto-Submit: Raphael Kubo da Costa <kubo@igalia.com>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1399097}