CRC-32 computation is stateless and the p slice does not get stored
anywhere. Thus, we mark the assembly functions as noescape so that
it doesn't believe that p leaks in:
	func Update(crc uint32, tab *Table, p []byte) uint32

Before:
	./crc32.go:153: leaking param: p

After:
	./crc32.go:153: Update p does not escape

Change-Id: I52ba35b6cc544fff724327140e0c27898431d1dc
Reviewed-on: https://go-review.googlesource.com/17069Reviewed-by: Russ Cox <rsc@golang.org>