Move the ELF32 and ELF64 structure definitions into their own files so
they can be reused when vDSO support is added for other architectures.

Change-Id: Id0171b4e5cea4add8635743c881e3bf3469597af
Reviewed-on: https://go-review.googlesource.com/93995
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>