Enable LLD_IS_LD by default on armv7

lld should now be a usable linker for armv7, and is already used as the
bootstrap linker (for linking the kernel and userland). Also enable as
the system linker now (/usr/bin/ld) for further testing and evaluation.
(This change will be reverted in case of unexpected fallout.)

Approved by: manu
Sponsored by: The FreeBSD Foundation