runtime: avoid dependence on main symbol

runtime: avoid dependence on main symbol

For -buildmode=plugin, this lets the linker drop the main.main symbol
out of the binary while including most of the runtime.

(In the future it should be possible to drop the entire runtime
package from plugins.)

Change-Id: I3e7a024ddf5cc945e3d8b84bf37a0b7cb2a00eb6
Reviewed-on: https://go-review.googlesource.com/27821
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>