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

[markj]

Summary

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.

[domagoj.stolfa_gmail.com]

sys/cddl/dev/dtrace/aarch64/dtrace_isa.c

257

Is it maybe worth KASSERTing alignment of tf_sp here?

[markj]

sys/cddl/dev/dtrace/aarch64/dtrace_isa.c

257

A KASSERT is probably too strong, but we should set a dtrace error flag if the alignment isn't right.

[domagoj.stolfa_gmail.com]

sys/cddl/dev/dtrace/aarch64/dtrace_isa.c

257

Yeah, that sounds like a better idea :).

[markj]

Check alignment. Reduce the amount of casting we do.