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

[arrowd]

Summary

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 symlink, so I omitted them.

Test Plan

With pkg-devel-2.0.99.5:

• emulators/libc6-shim does not provide anymore, as it's expected
• benchmarks/filebench a small port with USE_LDCONFIG=somedir provides correctly

[bapt]

I think this is exactly what we need, we would require 2 exp-run:
1 with regular pkg to ensure we are not breaking anything
1 with WITH_PKG=devel to ensure it works as expected