Enable lld as the system linker by default on amd64

The migration to LLVM's lld linker has been in progress for quite some
time - about three years ago I opened an upstream LLVM meta-bug to track
issues using lld as FreeBSD's linker, and about 1.5 years ago requested
the first exp-run with lld as the system linker.

As of r327783 we enabled LLD_BOOTSTRAP by default on amd64, using lld as
the linker to link the kernel and world, but GNU ld was still installed
as /usr/bin/ld.

The vast majority of issues observed when building ports with lld as the
system linker have now been solved, so set LLD_IS_LD by default on amd64
and install lld as /usr/bin/ld. A small number of port failures remain
and these will be addressed in the near future.

Thanks to antoine@ for handling the exp-runs, krion@ for investigating
many port failures and adding LLD_UNSAFE or other fixes or workarounds,
and everyone who helped investigate, fix or tag ports.

PR: 214864 (exp-run)
Sponsored by: The FreeBSD Foundation