8l/6l: new -Hwindowsgui flag allows to build windows gui pe

Fixes #1516.

R=rsc, mattn
CC=golang-dev
https://golang.org/cl/4214041

src/cmd/6l/obj.c

@@ -51,6 +51,7 @@ Header headers[] = {
    "linux", Hlinux,
    "freebsd", Hfreebsd,
    "windows", Hwindows,
+   "windowsgui", Hwindows,
    0, 0
 };
 

src/cmd/8l/obj.c

@@ -57,6 +57,7 @@ Header headers[] = {
    "nacl", Hnacl,
    "freebsd", Hfreebsd,
    "windows", Hwindows,
+   "windowsgui", Hwindows,
    "tiny", Htiny,
    0, 0
 };

src/cmd/ld/lib.c

@@ -1271,8 +1271,10 @@ headtype(char *name)
 	int i;
 
 	for(i=0; headers[i].name; i++)
-		if(strcmp(name, headers[i].name) == 0)
+		if(strcmp(name, headers[i].name) == 0) {
+			headstring = headers[i].name;
 			return headers[i].val;
+		}
 	fprint(2, "unknown header type -H %s\n", name);
 	errorexit();
 	return -1;  // not reached

src/cmd/ld/lib.h

@@ -237,5 +237,7 @@ struct Header {
 	int val;
 };
 
+EXTERN	char*	headstring;
 extern	Header	headers[];
+
 int	headtype(char*);

src/cmd/ld/pe.c

@@ -526,7 +526,10 @@ asmbpe(void)
 	set(MinorSubsystemVersion, 0);
 	set(SizeOfImage, nextsectoff);
 	set(SizeOfHeaders, PEFILEHEADR);
-	set(Subsystem, 3);	// WINDOWS_CUI
+	if(strcmp(headstring, "windowsgui") == 0)
+		set(Subsystem, IMAGE_SUBSYSTEM_WINDOWS_GUI);
+	else
+		set(Subsystem, IMAGE_SUBSYSTEM_WINDOWS_CUI);
 	set(SizeOfStackReserve, 0x0040000);
 	set(SizeOfStackCommit, 0x00001000);
 	set(SizeOfHeapReserve, 0x00100000);

src/cmd/ld/pe.h

@@ -131,6 +131,9 @@ enum {
 	IMAGE_DIRECTORY_ENTRY_IAT = 12,
 	IMAGE_DIRECTORY_ENTRY_DELAY_IMPORT = 13,
 	IMAGE_DIRECTORY_ENTRY_COM_DESCRIPTOR = 14,
+
+	IMAGE_SUBSYSTEM_WINDOWS_GUI = 2,
+	IMAGE_SUBSYSTEM_WINDOWS_CUI = 3,
 };
 
 void peinit(void);

src/pkg/exp/wingui/Makefile

@@ -6,6 +6,8 @@ GOOS=windows
 
 include ../../../Make.inc
 
+LD:=$(LD) -Hwindowsgui
+
 TARG=wingui
 
 GOFILES=\