cmd/link,compress/zip,image/png: use binary.{Big,Little}Endian methods

Use the binary.{Big,Little}Endian integer encoding methods rather than
variations found in local implementations. The functions in
the binary package have been tested to ensure they inline correctly and
don't add unnecessary bounds checking.

Change-Id: Ie10111ca6edb7c11e8e5e21c58a5748ae99b7f87
Reviewed-on: https://go-review.googlesource.com/134375


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>