Make Perl link all .so it builds with libperl.so.

It makes upgrading from one Perl major version to another way easier. For
binary package users, it means pkg upgrade will detect the libperl.so.x.yy
change, and reinstall the affected packages. For users using ports, it will
save rebuild time as it's easier to detect what ports really need to be
rebuilt.

PR: 195821
Differential Revision: https://reviews.freebsd.org/D1241
Submitted by: mat
Reviewed by: antoine, bdrewery
Exp-run by: antoine
With hat: perl
Sponsored by: Absolight