RFC: Switching armv7 to WITH_LLD_IS_LD for testing

[Ed Maste]

For amd64 I requested (and for i386 continue to request) multiple
exp-runs with lld as the system linker (/usr/bin/ld) before switching
the default in src.conf.5.

I'd like to switch armv7 to WITH_LLD_IS_LD; unfortunately we don't
have an easy way to execute an exp-run for arm (armv7 specifically).
If there's no objection I would like to enable WITH_LLD_IS_LD for
armv7, observe a cycle of ports build, with a plan to leave it enabled
if the build is largely successful, or revert if not.

Comments?