dtrace: remove ifdef around dtrace_instr_size()

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.

Reviewed by: markj
Approved by: markj (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39948