Run it right before main.init.
There is still some runtime initialization that
happens before runtime.init, and some of that
may call into Go code (for example to acquire locks)
so this timing is not perfect, but I believe it is the
best we can do.

This came up because global variables intialized
to func values are done in the generated init code,
not in the linker.

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews, iant, khr, r
https://golang.org/cl/135210043