Handle functions that use a nop in the arm64 fbt

To trace leaf asm functions we can insert a single nop instruction as
the first instruction in a function and trigger off this.

Reviewed by: gnn
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D28132

(cherry picked from commit 28d945204ea1014d7de6906af8470ed8b3311335)