Provide libdl.

Create libdl.so.1 as a filter for libc.so.7 which exports public dl*
functions. The functions are resolved from the rtld instead, the goal
of creating library is to avoid errors from the static linker due to
missed libdl. For static binaries, an empty .o is compiled into
libdl.a so that static binaries still get dl stubs from libc.a.

Right now lld cannot create filter objects, disable libdl on arm64
when binutils are not used.

Reviewed by: bdrewery, dim (previos version); emaste
Exp run: PR 220525, done by antoine
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D11504