dtrace: remove ifdef around dtrace_instr_size()

[christos]

Summary

Since D38825 is going to be using dtrace_instr_size() unconditionally,
architectures that are not included in the #ifdef won't be able to compile
libdtrace. This was tested on an ARM64 build. If the #ifdef is removed,
libdtrace can be compiled with no problems, otherwise it fails at libdtrace.

The current diff depends on D39871.

[markj]

Approved.