Enable ld.lld as bootstrap linker by default on amd64

For some time we have been planning to migrate to LLVM's lld linker.
Having a man page was the last blocking issue for using ld.lld to link
the base system kernel + userland, now addressed by r327770.  Link the
kernel and userland libraries and binaries with ld.lld by default, for
additional test coverage.

This has been a long time in the making.  On 2013-04-13 I submitted an
upstream tracking issue in LLVM PR 23214: [META] Using LLD as FreeBSD's
system linker.  Since then 85 individual issues were identified, and
submitted as dependencies.  These have been addressed along with two
and a half years of other lld development and improvement.

I'd like to express deep gratitude to upstream lld developers Rui
Ueyama, Rafael Espindola, George Rimar and Davide Italiano.  They put in
substantial effort in addressing the issues we found affecting
FreeBSD/amd64.

To revert to using ld.bfd as the bootstrap linker, in /etc/src.conf set

    WITHOUT_LLD_BOOTSTRAP=yes

If you need to set this, please follow up with a PR or post to the
freebsd-toolchain mailing list explaining how default WITH_LLD_BOOTSTRAP
failed for your use case.

Note that GNU ld.bfd is still installed as /usr/bin/ld, and will still
be used for linking ports.  ld.lld can be installed as /usr/bin/ld by
setting in /etc/src.conf

    WITH_LLD_IS_LLD=yes

A followup commit will set WITH_LLD_IS_LD by default, possibly after
Clang/LLVM/lld 6.0 is merged to FreeBSD.

Release notes:	Yes
Sponsored by:	The FreeBSD Foundation

share/mk/src.opts.mk

@@ -253,6 +253,9 @@ __DEFAULT_NO_OPTIONS+=LLVM_LIBUNWIND
 .endif
 .if ${__T} == "aarch64"
 __DEFAULT_YES_OPTIONS+=LLD_BOOTSTRAP LLD_IS_LD
+.elif ${__T} == "amd64"
+__DEFAULT_YES_OPTIONS+=LLD_BOOTSTRAP
+__DEFAULT_NO_OPTIONS+=LLD_IS_LD
 .else
 __DEFAULT_NO_OPTIONS+=LLD_BOOTSTRAP LLD_IS_LD
 .endif