build: Include $host_cpu in tapset directory

SystemTap tapsets are architecture-specific, as they include the full
path to the .so file for each probe they reference. Hence, we should
install them in an architecture-specific path, or multiarch systems will
suffer from collisions between them.

A better long-term solution, using $libdir rather than the
non-architecture-specific $datadir, is under discussion upstream:
https://sourceware.org/bugzilla/show_bug.cgi?id=20264; but this will do
for now.

https://bugzilla.gnome.org/show_bug.cgi?id=662802