syscall: fix dragonfly/amd64 assembly argument sizes

This is preliminary work to unifying the
unix amd64 assembly implementations,
which is preliminary work to making the
assembly vet-friendly.

Updates #11041

Change-Id: Ic64985124f8fb86cc08898be2ec7fca972ced4ca
Reviewed-on: https://go-review.googlesource.com/25113
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/asm_dragonfly_amd64.s

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// TODO(rsc): Rewrite all nn(SP) references into name+(nn-8)(FP)
+// so that go vet can check that they are correct.
+
 #include "textflag.h"
 #include "funcdata.h"
 
@@ -14,7 +17,7 @@
 // func Syscall9(trap int64, a1, a2, a3, a4, a5, a6, a7, a8, a9 int64) (r1, r2, err int64)
 // Trap # in AX, args in DI SI DX, return in AX DX
 
-TEXT	·Syscall(SB),NOSPLIT,$0-64
+TEXT	·Syscall(SB),NOSPLIT,$0-56
 	CALL	runtime·entersyscall(SB)
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
@@ -37,7 +40,7 @@ ok:
 	CALL	runtime·exitsyscall(SB)
 	RET
 
-TEXT	·Syscall6(SB),NOSPLIT,$0-88
+TEXT	·Syscall6(SB),NOSPLIT,$0-80
 	CALL	runtime·entersyscall(SB)
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
@@ -60,7 +63,7 @@ ok6:
 	CALL	runtime·exitsyscall(SB)
 	RET
 
-TEXT	·Syscall9(SB),NOSPLIT,$0-112
+TEXT	·Syscall9(SB),NOSPLIT,$0-104
 	CALL	runtime·entersyscall(SB)
 	MOVQ	8(SP), AX
 	MOVQ	16(SP), DI
@@ -93,7 +96,7 @@ ok9:
 	CALL	runtime·exitsyscall(SB)
 	RET
 
-TEXT ·RawSyscall(SB),NOSPLIT,$0-64
+TEXT ·RawSyscall(SB),NOSPLIT,$0-56
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX
@@ -113,7 +116,7 @@ ok1:
 	MOVQ	$0, 56(SP)	// errno
 	RET
 
-TEXT	·RawSyscall6(SB),NOSPLIT,$0-88
+TEXT	·RawSyscall6(SB),NOSPLIT,$0-80
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX