chacha20poly1305: Avo port of chacha20poly1305_amd64.s

This implementation utilizes the same registers found in the reference
implementation, aiming to produce a minimal semantic diff between the
Avo-generated output and the original hand-written assembly.

To verify the Avo implementation, the reference and Avo-generated
assembly files are fed to `go tool asm`, capturing the debug output into
corresponding temp files. The debug output contains supplementary
metadata (line numbers, instruction offsets, and source file references)
that must be removed in order to obtain a semantic diff of the two
files. This is accomplished via a small utility script written in awk.

Parameter metadata not found in the reference assembly file has been
added, leading to a diff on the lines where those symbols are
referenced.

Commands used to verify Avo output:

GOROOT=$(go env GOROOT)
ASM_PATH="chacha20poly1305/chacha20poly1305_amd64.s"
REFERENCE="b2d3a6a4b4d36521cd7f653879cf6981e7c5c340"

go tool asm -o /dev/null -I "$GOROOT"/src/runtime -debug \
  <(git cat-file -p "$REFERENCE:$ASM_PATH") \
  > /tmp/reference.s

go tool asm -o /dev/null -I "$GOROOT"/src/runtime -debug \
  "$ASM_PATH" \
  > /tmp/avo.s

normalize(){
  awk '{
    $1=$2=$3="";
    print substr($0,4)
  }'
}

diff <(normalize < /tmp/reference.s) <(normalize < /tmp/avo.s)

155,157c155,157
< MOVQ dst(FP), DI
< MOVQ key+24(FP), R8
< MOVQ src+48(FP), SI
---
> MOVQ dst_base(FP), DI
> MOVQ key_base+24(FP), R8
> MOVQ src_base+48(FP), SI
159c159
< MOVQ ad+72(FP), CX
---
> MOVQ ad_base+72(FP), CX
4684,4686c4684,4686
< MOVQ dst(FP), DI
< MOVQ key+24(FP), R8
< MOVQ src+48(FP), SI
---
> MOVQ dst_base(FP), DI
> MOVQ key_base+24(FP), R8
> MOVQ src_base+48(FP), SI
4688c4688
< MOVQ ad+72(FP), CX
---
> MOVQ ad_base+72(FP), CX

Change-Id: Ia3a8e70b7440944ee739499c41ddceb70e054ef9
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/601442
Reviewed-by: Filippo Valsorda <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>