Android's libc doesn't provide access to auxv, so currently the Go
runtime synthesizes a fake, minimal auxv when loaded as a library on
Android. This used to be sufficient, but now we depend on auxv to
retrieve the system physical page size and panic if we can't retrieve
it.

Fix this by falling back to reading auxv from /proc/self/auxv if the
loader-provided auxv is empty and removing the synthetic auxv vectors.

Fixes #18041.

Change-Id: Ia2ec2c764a6609331494a5d359032c56cbb83482
Reviewed-on: https://go-review.googlesource.com/33652
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>