crypto: avoid escaping Hash.Sum on generic architectures

For architectures without a specialized implementation (e.g. arm),
the generic implementation allocates because it does:

	var block = blockGeneric

which causes the compiler to give up trying to analyze block
even though it is technically only ever one implementation.
Instead of a variable, declare a function that wraps blockGeneric.

We apply this fix to md5, sha1, and sha256,
while sha512 already had the equivalent change.
We add a test to all hashing packages to ensure no allocations.

Credit goes to Cuong Manh Le for more specifically identifying
the problem and Keith Randal for suggesting a concrete solution.

Fixes #48055

Change-Id: I1a6a2e028038e051c83fd72b10a8bf4d210df57d
Reviewed-on: https://go-review.googlesource.com/c/go/+/346209


Trust: Joe Tsai <joetsai@digital-static.net>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>