dtrace: Add a partial implementation of dtrace_getarg() on arm64

For invop providers (i.e., fbt and kinst) we can simply reach into the
invop trapframe to fetch argument registers for arguments 0-7; for
argument 8 and beyond we have to read the value off of the stack.

Reviewed by: Domagoj Stolfa, avg
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D45649