Handle functions that use a nop in the arm64 fbt

[andrew]

Summary

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

Sponsored by: Innovate UK

[andrew]

Tested with dtrace -n ":::". This will trigger on memcpy as called by dtrace_getnanouptime, however it seems to be inlined by the in tree clang.