cmd/ld: always do external link for -linkmode=external

There are tests in run.bash for -linkmode=external.

Fixes #5238.

R=golang-dev, bradfitz, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/8716044

src/cmd/ld/lib.c

@@ -45,6 +45,10 @@ int	nlibdir = 0;
 static int	maxlibdir = 0;
 static int	cout = -1;
 
+// Set if we see an object compiled by the host compiler that is not
+// from a package that is known to support internal linking mode.
+static int	externalobj = 0;
+
 static	void	hostlinksetup(void);
 
 char*	goroot;
@@ -295,6 +299,19 @@ loadlib(void)
 		loadinternal("math");
 	if(flag_race)
 		loadinternal("runtime/race");
+	if(linkmode == LinkExternal) {
+		// This indicates a user requested -linkmode=external.
+		// The startup code uses an import of runtime/cgo to decide
+		// whether to initialize the TLS.  So give it one.  This could
+		// be handled differently but it's an unusual case.
+		loadinternal("runtime/cgo");
+		// Pretend that we really imported the package.
+		// This will do no harm if we did in fact import it.
+		s = lookup("go.importpath.runtime/cgo.", 0);
+		s->type = SDATA;
+		s->dupok = 1;
+		s->reachable = 1;
+	}
 
 	for(i=0; i<libraryp; i++) {
 		if(debug['v'])
@@ -303,13 +320,10 @@ loadlib(void)
 		objfile(library[i].file, library[i].pkg);
 	}
 	
-	if(linkmode == LinkExternal && !iscgo)
-		linkmode = LinkInternal;
-
-	// If we got this far in automatic mode, there were no
-	// cgo uses that suggest we need external mode.
-	// Switch to internal.
 	if(linkmode == LinkAuto) {
+		if(iscgo && externalobj)
+			linkmode = LinkExternal;
+		else
 			linkmode = LinkInternal;
 	}
 
@@ -532,8 +546,8 @@ ldhostobj(void (*ld)(Biobuf*, char*, int64, char*), Biobuf *f, char *pkg, int64
 		}
 	}
 
-	if(!isinternal && linkmode == LinkAuto)
-		linkmode = LinkExternal;
+	if(!isinternal)
+		externalobj = 1;
 
 	if(nhostobj >= mhostobj) {
 		if(mhostobj == 0)