For riscv kernel builds, add -N to LDFLAGS instead of replacing the
SYSTEM_LD variable. This avoids duplicating the contents of SYSTEM_LD
from kern.pre.mk just to add the -N flag to it. If the basic linker command
ever needs to be changed, this will be one less place that has to be found
and fixed.

Some testing by kp@ indicates that the -N flag may not be needed at all,
so a comment to that effect is also added, and the -N flag may be removed
in a followup commit.

Differential Revision: https://reviews.freebsd.org/D22920