This adds support for compressed ELF sections. This compression is
treated as a framing issue and hence the package APIs all
transparently decompress compressed sections. This requires some
subtlety for (*Section).Open, which returns an io.ReadSeeker: since
the decompressed data comes from an io.Reader, this commit introduces
a Reader-to-ReadSeeker adapter that is efficient for common uses of
Seek and does what it can otherwise.

Fixes #11773.

Change-Id: Ic0cb7255a85cadf4c1d15fb563d5a2e89dbd3c36
Reviewed-on: https://go-review.googlesource.com/17341Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Austin Clements <austin@google.com>