tools: Also create ${WORLDTMP}/legacy/usr/libdata/pkgonfig

This is needed for the next commit which will make libz a bootstrap
library as needed by ctfconvert. We could just not install the .pc file
as it's not needed, but that requires a per-library hack every time a
bootstrap library gains a .pc file, so this keeps bootstrap-tools
looking as much like a normal build as possible.

MFC after: 1 week

(cherry picked from commit 7ba31d58f00197b491828c981228faf6ae99a6c2)