On ARM, use the gold linker to avoid copy relocations.
https://sourceware.org/bugzilla/show_bug.cgi?id=19962

Change-Id: Icf82a38d39495d4518812713b957a03a6652c728
Reviewed-on: https://go-review.googlesource.com/22141
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>