Follows http://golang.org/cl/8454, a similar CL for arm architectures.
This CL involves android-specific changes, namely, synthesizing
argv/auxv, as android doesn't provide those to the init functions.

This code is based on crawshaw@ android code in golang.org/x/mobile.

Change-Id: I32364efbb2662e80270a99bd7dfb1d0421b5417d
Reviewed-on: https://go-review.googlesource.com/8457Reviewed-by: David Crawshaw <crawshaw@golang.org>