cgo: enable cgo on dragonfly

src/cmd/ld/lib.c

 // Derived from Inferno utils/6l/obj.c and utils/6l/span.c
 // http://code.google.com/p/inferno-os/source/browse/utils/6l/obj.c
 // http://code.google.com/p/inferno-os/source/browse/utils/6l/span.c
 //
 //      Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
 //      Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
 //      Portions Copyright © 1997-1999 Vita Nuova Limited
 //      Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
 //      Portions Copyright © 2004,2006 Bruce Ellis
 //      Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
@@ skipping 567 matching lines @@
         for(i=0; i<nelem(internalpkg); i++) {
                 if(strcmp(pkg, internalpkg[i]) == 0) {
                         isinternal = 1;
                         break;
                 }
         }
 
         if(!isinternal)
                 externalobj = 1;
 
+        // DragonFly declares errno with __thread, which results in a symbol
+        // type of R_386_TLS_GD or R_X86_64_TLSGD. The Go linker does not
+        // currently know how to handle TLS relocations, hence we have to
+        // force external linking for any libraries that link in code that
+        // uses errno. This can be removed if the Go linker ever supports
+        // these relocation types.
+        if(HEADTYPE == Hdragonfly)
+        if(strcmp(pkg, "net") == 0 || strcmp(pkg, "os/user") == 0)
+                externalobj = 1;
+
         if(nhostobj >= mhostobj) {
                 if(mhostobj == 0)
                         mhostobj = 16;
                 else
                         mhostobj *= 2;
                 hostobj = erealloc(hostobj, mhostobj*sizeof hostobj[0]);
         }
         h = &hostobj[nhostobj++];
         h->ld = ld;
         h->pkg = estrdup(pkg);
@@ skipping 1916 matching lines @@
         symgrow(ftab, start+(nhistfile+1)*4);
         setuint32(ftab, start, nhistfile);
         for(s = filesyms; s != S; s = s->next)
                 setuint32(ftab, start + s->value*4, ftabaddstring(ftab, s->name));
 
         ftab->size = ftab->np;
 ········
         if(debug['v'])
                 Bprint(&bso, "%5.2f pclntab=%lld bytes, funcdata total %lld bytes\n", cputime(), (vlong)ftab->size, (vlong)funcdata_bytes);
 }·······