riscv: Fix copyin/copyout

r343275 introduced a performance optimisation to the copyin/copyout
routines by attempting to copy word-per-word rather than byte-per-byte
where possible.

This optimisation failed to account for cases where the buffer is longer
than XLEN_BYTES, but due to misalignment does not not allow for any
word-sized copies. E.g. a 9 byte buffer (with XLEN_BYTES == 8) which is
misaligned by 2 bytes. The code nevertheless did a single full-word
copy, which meant we copied too much data. This potentially clobbered
other data.

This is most easily demonstrated by a simple sysctl -a.

Fix it by not assuming that we'll always have at least one full-word
copy to do, but instead checking the remaining length first.

Reviewed by: markj@, mhorne@, br@ (previous version)
MFC after: 1 week
Sponsored by: Axiado
Differential Revision: https://reviews.freebsd.org/D21100