Enable lld as the system linker by default on amd64

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