Decode AT_PAGESZ to determine physPageSize and AT_SUN_EXECNAME for
os.Executable.

Change-Id: I6ff774ad9d76c68fc61eb307df58217c17fd578d
Reviewed-on: https://go-review.googlesource.com/104375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>