runtime: change amd64 startup convention

Now the default startup is that the program begins at _rt0_amd64_$GOOS,
which sets DI = argc, SI = argv and jumps to _rt0_amd64.

This makes the _rt0_amd64 entry match the expected semantics for
the standard C "main" function, which we can now provide for use when
linking against a standard C library.

R=golang-dev, devon.odell, minux.ma
CC=golang-dev
https://golang.org/cl/7525043

src/pkg/runtime/asm_amd64.s

@@ -6,8 +6,8 @@
 
 TEXT _rt0_amd64(SB),7,$-8
 	// copy arguments forward on an even stack
-	MOVQ	0(DI), AX		// argc
-	LEAQ	8(DI), BX		// argv
+	MOVQ	DI, AX		// argc
+	MOVQ	SI, BX		// argv
 	SUBQ	$(4*8+7), SP		// 2args 2auto
 	ANDQ	$~15, SP
 	MOVQ	AX, 16(SP)

src/pkg/runtime/rt0_darwin_amd64.s

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Darwin and Linux use the same linkage to main
-
 TEXT _rt0_amd64_darwin(SB),7,$-8
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
 	MOVQ	$_rt0_amd64(SB), AX
-	MOVQ	SP, DI
 	JMP	AX

src/pkg/runtime/rt0_freebsd_amd64.s

@@ -2,8 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Darwin and Linux use the same linkage to main
-
 TEXT _rt0_amd64_freebsd(SB),7,$-8
-	MOVQ	$_rt0_amd64(SB), DX
-	JMP	DX
+	LEAQ	8(DI), SI // argv
+	MOVQ	0(DI), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
+	MOVQ	$_rt0_amd64(SB), AX
+	JMP	AX

src/pkg/runtime/rt0_linux_amd64.s

@@ -2,9 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Darwin and Linux use the same linkage to main
-
 TEXT _rt0_amd64_linux(SB),7,$-8
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
 	MOVQ	$_rt0_amd64(SB), AX
-	MOVQ	SP, DI
 	JMP	AX

src/pkg/runtime/rt0_netbsd_amd64.s

@@ -2,7 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_amd64_netbsd(SB),7,$-8
-	MOVQ	$_rt0_amd64(SB), DX
-	MOVQ	SP, DI
-	JMP	DX
+TEXT _rt0_amd64_openbsd(SB),7,$-8
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
+	MOVQ	$_rt0_amd64(SB), AX
+	JMP	AX

src/pkg/runtime/rt0_openbsd_amd64.s

@@ -3,6 +3,11 @@
 // license that can be found in the LICENSE file.
 
 TEXT _rt0_amd64_openbsd(SB),7,$-8
-	MOVQ	$_rt0_amd64(SB), DX
-	MOVQ	SP, DI
-	JMP	DX
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
+	MOVQ	$_rt0_amd64(SB), AX
+	JMP	AX

src/pkg/runtime/rt0_plan9_amd64.s

@@ -2,9 +2,10 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_amd64_plan9(SB),7, $0
+TEXT _rt0_amd64_plan9(SB),7,$-8
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
 	MOVQ	$_rt0_amd64(SB), AX
-	MOVQ	SP, DI
 	JMP	AX
 
 DATA runtime·isplan9(SB)/4, $1

src/pkg/runtime/rt0_windows_amd64.s

@@ -4,9 +4,14 @@
 
 #include "zasm_GOOS_GOARCH.h"
 
-TEXT	_rt0_amd64_windows(SB),7,$-8
+TEXT _rt0_amd64_darwin(SB),7,$-8
+	LEAQ	8(SP), SI // argv
+	MOVQ	0(SP), DI // argc
+	MOVQ	$main(SB), AX
+	JMP	AX
+
+TEXT main(SB),7,$-8
 	MOVQ	$_rt0_amd64(SB), AX
-	MOVQ	SP, DI
 	JMP	AX
 
 DATA  runtime·iswindows(SB)/4, $1