poly1305: fix stack handling in sum_arm.s
poly1305: fix stack handling in sum_arm.s

Up till now, sum_arm.s was working only because of luck. It was written
assuming that it had stack space below the current stack pointer, but Go
decrements the stack pointer in the function prelude, so it was just
writing off the end of the stack.

This change fixes the stack manipulation so that it only writes within
the bounds.

Fixes golang/go#17499.

Change-Id: I1951b3344c21f6bd6ade79da8b96dd1bb68180db
Reviewed-on: https://go-review.googlesource.com/31443
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/crypto,ScriptRock/crypto,ScriptRock/crypto,golang/crypto,golang/crypto