Simplify the RISC-V kernel linker invocation

Remove our custom SYSTEM_LD definition. This generates program headers
that are more consistent with other architectures, and more importantly,
are in line with what loader(8) expects when loading a kernel.

As noted in https://reviews.freebsd.org/D22920, there is no apparent
reason why the kernel would need a writable text segment, so removal of
the -N flag isn't likely to cause issue.

Reviewed by: kp, br
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D24909