Install working pkgconfig .pc files for compat libraries

The default ones are install them to /usr/libdata/pkgconfig, and we can't
use this path for compat libraries, so we use /usr/lib<suffix>/pkgconfigi here.

Test Plan: grep -rn libdir= ./usr/lib32/pkgconfig/*.pc
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34939