Use lld as the bootstrap linker on armv7 by default

As of r336972 lld is capable linking the armv7 kernel and userland,
so enable it by default.

PR: 229050
Reviewed by: kevans
Tested by: kevans
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D16528