salsa20/salsa: fix amd64 assembly not to smash SP
salsa20/salsa: fix amd64 assembly not to smash SP

For golang/go#44269.

Change-Id: Ica352261d696317addbdd422d4cde5bf07fef839
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/292050
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Trust: Jason A. Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
Reviewed-by: Jason A. Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>

golang/crypto,golang/crypto,golang/crypto