Add infrastructure to install lld as the default linker. The old GNU linker

Add infrastructure to install lld as the default linker.  The old GNU linker
will be installed as /usr/bin/ld.bfd on supported systems.  This allows
users to fall back on the old linker by using the -fuse-ld=bfd option on
systems where lld is the default linker.

Switch armv7 to use lld as the default linker.  On arm64 we already use lld
as the default linker.  Other platforms will keep using the GNU linker for
now.

ok patrick@, deraadt@, phessler@