cmd/gc, cmd/5g, cmd/6g, cmd/8g: introduce linkarchinit and add amd64p32 support

Replaces CL 70000043.

Introduce linkarchinit() from cmd/ld.

For cmd/6g, switch to the amd64p32 linker model if we are building under nacl/amd64p32.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/71330045

src/cmd/5g/galign.c

@@ -10,6 +10,11 @@ int	thechar	= '5';
 char*	thestring	= "arm";
 LinkArch*	thelinkarch = &linkarm;
 
+void
+linkarchinit(void)
+{
+}
+
 vlong MAXWIDTH = (1LL<<32) - 1;
 
 /*

src/cmd/6g/galign.c

@@ -10,6 +10,13 @@ int	thechar	= '6';
 char*	thestring	= "amd64";
 LinkArch*	thelinkarch = &linkamd64;
 
+void
+linkarchinit(void)
+{
+	if(strcmp(getgoarch(), "amd64p32") == 0)
+		thelinkarch = &linkamd64p32;
+}
+
 vlong MAXWIDTH = 1LL<<50;
 
 int	addptr = AADDQ;

src/cmd/8g/galign.c

@@ -10,6 +10,11 @@ int	thechar	= '8';
 char*	thestring	= "386";
 LinkArch*	thelinkarch = &link386;
 
+void
+linkarchinit(void)
+{
+}
+
 vlong MAXWIDTH = (1LL<<32) - 1;
 
 /*

src/cmd/gc/go.h

@@ -1505,6 +1505,7 @@ Prog*	gjmp(Prog*);
 void	gused(Node*);
 void	movelarge(NodeList*);
 int	isfat(Type*);
+void	linkarchinit(void);
 void	liveness(Node*, Prog*, Sym*, Sym*, Sym*);
 void	markautoused(Prog*);
 Plist*	newplist(void);

src/cmd/gc/lex.c

@@ -212,7 +212,14 @@ main(int argc, char *argv[])
 #ifdef	PLAN9
 	notify(catcher);
 #endif
+	// Allow GOARCH=thestring or GOARCH=thestringsuffix,
+	// but not other values.	
+	p = getgoarch();
+	if(strncmp(p, thestring, strlen(thestring)) != 0)
+		fatal("cannot use %cg with GOARCH=%s", thechar, p);
+	goarch = p;
 
+	linkarchinit();
 	ctxt = linknew(thelinkarch);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;
@@ -259,13 +266,6 @@ main(int argc, char *argv[])
 	goroot = getgoroot();
 	goos = getgoos();
 
-	// Allow GOARCH=thestring or GOARCH=thestringsuffix,
-	// but not other values.	
-	p = getgoarch();
-	if(strncmp(p, thestring, strlen(thestring)) != 0)
-		fatal("cannot use %cg with GOARCH=%s", thechar, p);
-	goarch = p;
-	
 	nacl = strcmp(goos, "nacl") == 0;
 	if(nacl)
 		flag_largemodel = 1;