crossdev: fix sysroot handling in pkg-config wrapper

The pkg-config wrapper re-implemented SYSROOT support because it didn't
work correctly in pkg-config before 0.24. That was released years ago so
it is time to update, custom code broke when paths were outside of /usr
so libraries installed to /lib64 broke things.

With this fixed we don't need to generate our own wrappers.

Upstream bug: https://bugs.gentoo.org/show_bug.cgi?id=517530
Merge pull request #754 from marineam/dev

crossdev: fix sysroot handling in pkg-config wrapper