Already supported platforms are linux/amd64 and android/arm.

Running -buildmode=c-shared on linux/arm is equivalent to:
  -ldflags "-shared" -asmflags "-shared"

Change-Id: Ifdb267f1d6508157f236be912fa369440172d161
Reviewed-on: https://go-review.googlesource.com/8895Reviewed-by: David Crawshaw <crawshaw@golang.org>