crypto/sha512: fix stack size for previous change

In a recent change CL 388654 a function was updated so it
no longer needed stack space, but the TEXT statement was
not updated to reflect that change. This corrects that problem.

Change-Id: I9e60cebddae620788b1097ab7b39c47b323d1f62
Reviewed-on: https://go-review.googlesource.com/c/go/+/389674


Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>