Mk/bsd.port.mk: Setup SHLIB_PROVIDE_PATHS* vars for pkg

This changes picks a new pkg feature: https://github.com/freebsd/pkg/pull/2422

The idea behind the .if ${LINUX_DEFAULT} ... check comes from the Linux
ld.so(8) man page that says

(5) In the default path /lib, and then /usr/lib. (On some 64-bit

architectures, the default paths for 64-bit shared objects
are /lib64, and then /usr/lib64.)

CentOS 7 and Rocky Linux 9 that we have in Ports install 32 bit libraries
into /usr/lib and 64 bit libraries into /usr/lib64. /lib and /lib64 are
symlinks, so I omitted them.

Differential Revision: https://reviews.freebsd.org/D49136
Approved by: bapt