The new linker will disallow this on arm
(it is already disallowed on amd64 and 386)
in order to be able to lay out each function
separately.

The restriction is only for jumps into the middle
of a function; jumps to the beginning of a function
remain fine.

Prereq for linker cleanup (golang.org/s/go13linker).

R=iant, r, minux.ma
CC=golang-dev
https://golang.org/cl/35800043