runtime: change 386 startup convention

Now the default startup is that the program begins at _rt0_386_$GOOS,
which behaves as if calling main(argc, argv). Main jumps to _rt0_386.

This makes the _rt0_386 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.

386 analogue of https://golang.org/cl/7525043

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7551045

src/pkg/runtime/asm_386.s

@@ -6,8 +6,8 @@
 
 TEXT _rt0_386(SB),7,$0
 	// copy arguments forward on an even stack
-	MOVL	0(SP), AX		// argc
+	MOVL	argc+0(FP), AX
-	LEAL	4(SP), BX		// argv
+	MOVL	argv+4(FP), BX
 	SUBL	$128, SP		// plenty of scratch
 	ANDL	$~15, SP
 	MOVL	AX, 120(SP)		// save argc, argv away

src/pkg/runtime/rt0_darwin_386.s

@@ -2,7 +2,13 @@
 // 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_386_darwin(SB),7,$8
+	MOVL	8(SP), AX
+	LEAL	12(SP), BX
+	MOVL	AX, 0(SP)
+	MOVL	BX, 4(SP)
+	CALL	main(SB)
+	INT	$3
 
-TEXT _rt0_386_darwin(SB),7,$0
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)

src/pkg/runtime/rt0_freebsd_386.s

@@ -2,8 +2,13 @@
 // 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_386_freebsd(SB),7,$8
+	MOVL	8(SP), AX
+	LEAL	12(SP), BX
+	MOVL	AX, 0(SP)
+	MOVL	BX, 4(SP)
+	CALL	main(SB)
+	INT	$3
 
-TEXT _rt0_386_freebsd(SB),7,$0
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)
-

src/pkg/runtime/rt0_linux_386.s

@@ -2,10 +2,16 @@
 // 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_386_linux(SB),7,$8
-
+	MOVL	8(SP), AX
-TEXT _rt0_386_linux(SB),7,$0
+	LEAL	12(SP), BX
+	MOVL	AX, 0(SP)
+	MOVL	BX, 4(SP)
 	CALL	runtime·linux_setup_vdso(SB)
+	CALL	main(SB)
+	INT	$3
+
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)
 
 TEXT _fallback_vdso(SB),7,$0

src/pkg/runtime/rt0_netbsd_386.s

@@ -2,5 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_386_netbsd(SB),7,$0
+TEXT _rt0_386_netbsd(SB),7,$8
+	MOVL	8(SP), AX
+	LEAL	12(SP), BX
+	MOVL	AX, 0(SP)
+	MOVL	BX, 4(SP)
+	CALL	main(SB)
+	INT	$3
+
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)

src/pkg/runtime/rt0_openbsd_386.s

@@ -2,5 +2,13 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_386_openbsd(SB),7,$0
+TEXT _rt0_386_openbsd(SB),7,$8
+	MOVL	8(SP), AX
+	LEAL	12(SP), BX
+	MOVL	AX, 0(SP)
+	MOVL	BX, 4(SP)
+	CALL	main(SB)
+	INT	$3
+
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)

src/pkg/runtime/rt0_plan9_386.s

@@ -26,6 +26,13 @@ argv_fix:
 	LOOP	argv_fix
 	
 	CALL	runtime·asminit(SB)
+
+	MOVL	0(SP), AX
+	LEAL	4(SP), BX
+	PUSHL	BX
+	PUSHL	AX
+	PUSHL	$-1
+
 	JMP	_rt0_386(SB)
 
 DATA  runtime·isplan9(SB)/4, $1

src/pkg/runtime/rt0_windows_386.s

@@ -2,8 +2,17 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_386_windows(SB),7,$0
+TEXT _rt0_386_windows(SB),7,$12
+	MOVL	12(SP), AX
+	LEAL	16(SP), BX
+	MOVL	AX, 4(SP)
+	MOVL	BX, 8(SP)
+	MOVL	$-1, 0(SP) // return PC for main
+	JMP	main(SB)
+
+TEXT main(SB),7,$0
 	JMP	_rt0_386(SB)
 
+
 DATA  runtime·iswindows(SB)/4, $1
 GLOBL runtime·iswindows(SB), $4

src/pkg/runtime/signal_linux_386.c

@@ -155,9 +155,8 @@ extern uint32 runtime·_vdso;
 
 #pragma textflag 7
 void
-runtime·linux_setup_vdso(int32 argc, void *argv_list)
+runtime·linux_setup_vdso(int32 argc, byte **argv)
 {
-	byte **argv = &argv_list;
 	byte **envp;
 	uint32 *auxv;