build-sys: move systemd.pc from pkgconfiglibdir back into pkgconfigdatadir

build-sys: move systemd.pc from pkgconfiglibdir back into pkgconfigdatadir

The original idea of systemd.pc was to contain arch-independent system
and systemd information. By exposing libdir as part of the fields (added
in eb39a6239c631873db62f6a942e6cb3dab0a2db4), it started to carry
arch-dependent data, thus breaking multilib systems. It was then moved
to pkgconfiglibdir to deal with this (in
aec432c6134146e138124c4130be2ee89dca07fa), but actually the right
approach is to simply not include libdir in the .pc file at all.

THis patch hence more or less reverts both commits again, and moves the
.pc file back into pkgconfigdatadir.

As alternative for querying the systems primary libdir there's now
"systemd-path system-library-arch", hence a more correct alternative
exists for querying this variable from the .pc file.