krb5: Build pkgconfig files with libraries

[ivy]

Summary

For various reasons, trying to build 32-bit compatibility PCFILES
outside of a directory which is not a library doesn't work. Add a
new krb5/Makefile.pc with the build rule for .pc.in.pc, and use
this to build each pc file along with the library it's associated
with.

This means we automatically get 32-bit pcfiles in /usr/lib32, and
is arguably more correct anyway since if we don't build a library
for some reason, we also won't build its pcfiles.