syscall: superficial cleanup of amd64 unix assembly

This is preliminary work to unifying them.

Updates #11041

Change-Id: Ibe83da3d626f1da9e8888e26cedd3af2152b42e6
Reviewed-on: https://go-review.googlesource.com/25114
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

@@ -14,18 +14,18 @@
 
 // func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
 // func Syscall6(trap int64, a1, a2, a3, a4, a5, a6 int64) (r1, r2, err int64);
-// func Syscall9(trap int64, a1, a2, a3, a4, a5, a6, a7, a8, a9 int64) (r1, r2, err int64)
+// 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,$0-56
 	CALL	runtime·entersyscall(SB)
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX
 	MOVQ	$0, R10
 	MOVQ	$0, R8
 	MOVQ	$0, R9
-	MOVQ	8(SP), AX	// syscall entry
 	SYSCALL
 	JCC	ok
 	MOVQ	$-1, 40(SP)	// r1
@@ -42,13 +42,13 @@ ok:
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-80
 	CALL	runtime·entersyscall(SB)
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX
 	MOVQ	40(SP), R10
 	MOVQ	48(SP), R8
 	MOVQ	56(SP), R9
-	MOVQ	8(SP), AX	// syscall entry
 	SYSCALL
 	JCC	ok6
 	MOVQ	$-1, 64(SP)	// r1
@@ -65,7 +65,7 @@ ok6:
 
 TEXT	·Syscall9(SB),NOSPLIT,$0-104
 	CALL	runtime·entersyscall(SB)
-	MOVQ	8(SP), AX
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX

src/syscall/asm_freebsd_amd64.s

@@ -14,18 +14,18 @@
 
 // func Syscall(trap int64, a1, a2, a3 int64) (r1, r2, err int64);
 // func Syscall6(trap int64, a1, a2, a3, a4, a5, a6 int64) (r1, r2, err int64);
-// func Syscall9(trap int64, a1, a2, a3, a4, a5, a6, a7, a8, a9 int64) (r1, r2, err int64)
+// 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,$0-56
 	CALL	runtime·entersyscall(SB)
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX
 	MOVQ	$0, R10
 	MOVQ	$0, R8
 	MOVQ	$0, R9
-	MOVQ	8(SP), AX	// syscall entry
 	SYSCALL
 	JCC	ok
 	MOVQ	$-1, 40(SP)	// r1
@@ -42,13 +42,13 @@ ok:
 
 TEXT	·Syscall6(SB),NOSPLIT,$0-80
 	CALL	runtime·entersyscall(SB)
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX
 	MOVQ	40(SP), R10
 	MOVQ	48(SP), R8
 	MOVQ	56(SP), R9
-	MOVQ	8(SP), AX	// syscall entry
 	SYSCALL
 	JCC	ok6
 	MOVQ	$-1, 64(SP)	// r1
@@ -65,7 +65,7 @@ ok6:
 
 TEXT	·Syscall9(SB),NOSPLIT,$0-104
 	CALL	runtime·entersyscall(SB)
-	MOVQ	8(SP), AX
+	MOVQ	8(SP), AX	// syscall entry
 	MOVQ	16(SP), DI
 	MOVQ	24(SP), SI
 	MOVQ	32(SP), DX