cmd/6a: add support for nacl/amd64p32

Replaces CL 70000043.

Switch to the amd64p32 linker model if we are building under nacl/amd64p32.

No need to introduce linkarchinit() as 6a contains its own main() function.

LGTM=rsc
R=rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/72020043

src/cmd/6a/lex.c

@@ -63,6 +63,8 @@ Lconv(Fmt *fp)
 	return linklinefmt(ctxt, fp);
 }
 
+LinkArch*       thelinkarch = &linkamd64;
+
 void
 main(int argc, char *argv[])
 {
@@ -72,18 +74,20 @@ main(int argc, char *argv[])
 	thechar = '6';
 	thestring = "amd64";
 
-	ctxt = linknew(&linkamd64);
-	ctxt->diag = yyerror;
-	ctxt->bso = &bstdout;
-	Binit(&bstdout, 1, OWRITE);
-	listinit6();
-	fmtinstall('L', Lconv);
-
 	// Allow GOARCH=thestring or GOARCH=thestringsuffix,
 	// but not other values.	
 	p = getgoarch();
 	if(strncmp(p, thestring, strlen(thestring)) != 0)
 		sysfatal("cannot use %cc with GOARCH=%s", thechar, p);
+	if(strcmp(p, "amd64p32") == 0)
+		thelinkarch = &linkamd64p32;
+
+	ctxt = linknew(thelinkarch);
+	ctxt->diag = yyerror;
+	ctxt->bso = &bstdout;
+	Binit(&bstdout, 1, OWRITE);
+	listinit6();
+	fmtinstall('L', Lconv);
 
 	ensuresymb(NSYMB);
 	memset(debug, 0, sizeof(debug));