dtrace: correct profile aframes value for riscv

Experimentation shows this is the correct value; the dtrace/interrupt
handler frames are omitted, while the backtrace of the active thread is
recorded in its entirety.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37662

(cherry picked from commit 40fdda022a16ec1a5ec18e7111a8cb8c147aefaf)