Revert "runtime/internal/syscall: use ABIInternal for Syscall6 on amd64"

This reverts CL 401096. Grandparent CL 388477 breaks cmd/go
TestScript/cover_pkgall_runtime.

For #51087.
For #52472.

Change-Id: Ie82fe5f50975f66eb91fb0d01cd8bbbd0265eb4e
Reviewed-on: https://go-review.googlesource.com/c/go/+/401634
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/internal/syscall/asm_linux_amd64.s

@@ -6,42 +6,28 @@
 
 // func Syscall6(num, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, errno uintptr)
 //
-// We need to convert to the syscall ABI.
-//
-// arg | ABIInternal | Syscall
-// ---------------------------
-// num | AX          | AX
-// a1  | BX          | DI
-// a2  | CX          | SI
-// a3  | DI          | DX
-// a4  | SI          | R10
-// a5  | R8          | R8
-// a6  | R9          | R9
-//
-// r1  | AX          | AX
-// r2  | BX          | DX
-// err | CX          | part of AX
+// Syscall # in AX, args in DI SI DX R10 R8 R9, return in AX DX.
 //
 // Note that this differs from "standard" ABI convention, which would pass 4th
 // arg in CX, not R10.
-TEXT ·Syscall6<ABIInternal>(SB),NOSPLIT,$0
-	// a6 already in R9.
-	// a5 already in R8.
-	MOVQ	SI, R10 // a4
-	MOVQ	DI, DX  // a3
-	MOVQ	CX, SI  // a2
-	MOVQ	BX, DI  // a1
-	// num already in AX.
+TEXT ·Syscall6(SB),NOSPLIT,$0-80
+	MOVQ	num+0(FP), AX	// syscall entry
+	MOVQ	a1+8(FP), DI
+	MOVQ	a2+16(FP), SI
+	MOVQ	a3+24(FP), DX
+	MOVQ	a4+32(FP), R10
+	MOVQ	a5+40(FP), R8
+	MOVQ	a6+48(FP), R9
 	SYSCALL
 	CMPQ	AX, $0xfffffffffffff001
 	JLS	ok
+	MOVQ	$-1, r1+56(FP)
+	MOVQ	$0, r2+64(FP)
 	NEGQ	AX
-	MOVQ	AX, CX  // errno
-	MOVQ	$-1, AX // r1
-	MOVQ	$0, BX  // r2
+	MOVQ	AX, errno+72(FP)
 	RET
 ok:
-	// r1 already in AX.
-	MOVQ	DX, BX // r2
-	MOVQ	$0, CX // errno
+	MOVQ	AX, r1+56(FP)
+	MOVQ	DX, r2+64(FP)
+	MOVQ	$0, errno+72(FP)
 	RET