meson: install libsystemd-shared into rootpkglibdir

meson: install libsystemd-shared into rootpkglibdir

Introduce rootpkglibdir for installing libsystemd-{shared,core}.so.
The benefit over using rootlibexecdir is that this path can be
multiarch aware, i.e. this path can be architecture qualified.

This is something we'd like to make use of in Debian/Ubuntu to make
libsystemd-shared co-installable, e.g. for i386 the path would be
/usr/lib/i386-linux-gnu/systemd/libsystemd-shared-*.so and for amd64
/usr/lib/x86_64-linux-gnu/systemd/libsystemd-shared-*.so.
This will allow for example to install and run systemd-boot/i386 on an
amd64 host. It also simplifies/enables cross-building/bootstrapping.

For more infos about Multi-Arch see https://wiki.debian.org/Multiarch.

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=990547