kinst: fix memcpy() tracing crash

Tracing memcpy() would crash the kernel, because we'd also trace the
memcpy() calls from kinst_invop(). To fix this, introduce kinst_memcpy()
whose arguments are 'volatile', so that we avoid having the compiler
replace it with a regular memcpy().

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40284