Use TEXT pseudo-instruction to adjust SP instead of a SUB instruction
so that the assembler knows how to fill in the pcsp table and the frame
description entry correctly.

Updates #21569

Change-Id: I436c840b2af99bbb3042ecd38a7d7c1ab4d7372a
Reviewed-on: https://go-review.googlesource.com/70937
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>