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>

Android linker does not handle TLS for us. We set up the TLS slot
for g, as darwin/386,amd64 handle instead. This is disgusting and
fragile. We will eventually fix this ugly hack by taking advantage
of the recent TLS IE model implementation. (Instead of referencing
an GOT entry, make the code sequence look into the TLS variable that
holds the offset.)

The TLS slot for g in android/amd64 assumes a fixed offset from %fs.
See runtime/cgo/gcc_android_amd64.c for details.

For golang/go#10743

Change-Id: I1a3fc207946c665515f79026a56ea19134ede2dd
Reviewed-on: https://go-review.googlesource.com/15991
Reviewed-by: David Crawshaw <crawshaw@golang.org>