cmd/go: address DWARF linker issues with -buildmode=plugin on Darwin

Assorted fixups in the linker needed to enable turning back on
DWARF generation when building plugins for Darwin. Includes:

 - don't suppress import of runtime/cgo in the linker for
   Darwin if we are linking in plugin mode

 - in calcCompUnitRanges handle the case where we encounter
   linker-generated functions that have no associated Unit (and
   also have no DWARF)

 - generalize a guard in relocsym() include so as to avoid
   triggering a spurious error on go.info symbols in plugin mode

Updates #21647.
Updates #27502.

Change-Id: I317fea97bef2f3461e31498e63f9fd6d8b8f4b23
Reviewed-on: https://go-review.googlesource.com/c/go/+/182959
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/cmd/link/internal/ld/data.go

@@ -157,8 +157,8 @@ func relocsym(ctxt *Link, s *sym.Symbol) {
 		if r.Sym != nil && ((r.Sym.Type == sym.Sxxx && !r.Sym.Attr.VisibilityHidden()) || r.Sym.Type == sym.SXREF) {
 			// When putting the runtime but not main into a shared library
 			// these symbols are undefined and that's OK.
-			if ctxt.BuildMode == BuildModeShared {
-				if r.Sym.Name == "main.main" || r.Sym.Name == "main..inittask" {
+			if ctxt.BuildMode == BuildModeShared || ctxt.BuildMode == BuildModePlugin {
+				if r.Sym.Name == "main.main" || (ctxt.BuildMode != BuildModePlugin && r.Sym.Name == "main..inittask") {
 					r.Sym.Type = sym.SDYNIMPORT
 				} else if strings.HasPrefix(r.Sym.Name, "go.info.") {
 					// Skip go.info symbols. They are only needed to communicate

src/cmd/link/internal/ld/dwarf.go

@@ -944,6 +944,11 @@ func calcCompUnitRanges(ctxt *Link) {
 		if s.FuncInfo == nil {
 			continue
 		}
+		// Skip linker-created functions (ex: runtime.addmoduledata), since they
+		// don't have DWARF to begin with.
+		if s.Unit == nil {
+			continue
+		}
 		unit := s.Unit
 		// Update PC ranges.
 		//

src/cmd/link/internal/ld/lib.go

@@ -437,7 +437,7 @@ func (ctxt *Link) loadlib() {
 	// We now have enough information to determine the link mode.
 	determineLinkMode(ctxt)
 
-	if ctxt.LinkMode == LinkExternal && !iscgo && ctxt.LibraryByPkg["runtime/cgo"] == nil && !(objabi.GOOS == "darwin" && (ctxt.Arch.Family == sys.AMD64 || ctxt.Arch.Family == sys.I386)) {
+	if ctxt.LinkMode == LinkExternal && !iscgo && ctxt.LibraryByPkg["runtime/cgo"] == nil && !(objabi.GOOS == "darwin" && ctxt.BuildMode != BuildModePlugin && (ctxt.Arch.Family == sys.AMD64 || ctxt.Arch.Family == sys.I386)) {
 		// 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