cmd/cc, cmd/5c, cmd/6c, cmd/8c: introduce linkarchinit and add amd64p32 support

Replaces CL 70000043.

Introduce linkarchinit() from cmd/ld.

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

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/72010043

src/cmd/6c/txt.c

@@ -30,11 +30,18 @@
 
 #include "gc.h"
 
-LinkArch	*thelinkarch = &linkamd64;
-
 int thechar = '6';
 char *thestring = "amd64";
 
+LinkArch	*thelinkarch = &linkamd64;
+
+void
+linkarchinit(void)
+{
+	if(strcmp(getgoarch(), "amd64p32") == 0)
+		thelinkarch = &linkamd64p32;
+}
+
 void
 ginit(void)
 {

src/cmd/8c/txt.c

@@ -30,11 +30,17 @@
 
 #include "gc.h"
 
-LinkArch	*thelinkarch = &link386;
 
 int thechar = '8';
 char *thestring = "386";
 
+LinkArch	*thelinkarch = &link386;
+
+void
+linkarchinit(void)
+{
+}
+
 void
 ginit(void)
 {

src/cmd/cc/cc.h

@@ -795,6 +795,7 @@ int32	exreg(Type*);
 int32	align(int32, Type*, int, int32*);
 int32	maxround(int32, int32);
 int	hasdotdotdot(void);
+void    linkarchinit(void);
 
 extern	schar	ewidth[];
 

src/cmd/cc/lex.c

@@ -124,7 +124,7 @@ main(int argc, char *argv[])
 	p = getgoarch();
 	if(strncmp(p, thestring, strlen(thestring)) != 0)
 		sysfatal("cannot use %cc with GOARCH=%s", thechar, p);
-	if(strcmp(getgoarch(), "amd64p32") == 0) // must be before cinit
+	if(strcmp(p, "amd64p32") == 0) // must be before cinit
 		ewidth[TIND] = 4;
 		
 	nacl = strcmp(getgoos(), "nacl") == 0;
@@ -133,6 +133,7 @@ main(int argc, char *argv[])
 
 	quotefmtinstall(); // before cinit, which overrides %Q
 
+	linkarchinit();
 	ctxt = linknew(thelinkarch);
 	ctxt->diag = yyerror;
 	ctxt->bso = &bstdout;