libdrm data and drirc.d not brought in from Exherbo /usr/share

Exherbo has a variant of multiarch where shared libraries are in /usr/${gnu_tuple}/lib and architecture-independent data is in /usr/share.

The algorithm we use to guess the ${prefix} from the name of a shared library doesn't deal with this: we will strip off /lib, /lib32, /lib64 and /lib/${multiarch} suffixes, but not /${gnu_tuple}/lib. This means we don't make the host system's /usr/share/libdrm or /usr/share/drirc.d available in the container. Those are non-essential but helpful.

We should choose one of these:

• adjust that algorithm so we go from /usr/${gnu_tuple}/lib/libdrm.so.2 to /usr/share/libdrm, and similar for /usr/share/drirc.d

• hard-code /usr/share to be tried after anywhere else that we might look for those directories

/cc @denittis