cmd/link: load symbols from .syso in external link mode

Fix linking with a package having a .syso file in external link mode,
that would otherwise cause an error before executing the external
linker because it can't find symbols that are exported in the said
.syso file.

Fixes #33139

src/cmd/go/testdata/script/link_syso_issue33139.txt

@@ -0,0 +1,36 @@
+# Test that we can link a syso file that is embedded in a package, that is
+# referenced by an assembly stub.  See issue 33139.
+[!gc] stop
+[darwin] cc -c -o syso/test.syso syso/asm/test_darwin.s
+[!darwin] cc -c -o syso/test.syso syso/asm/test.s
+go build -ldflags='-linkmode=external' ./cmd/main.go
+
+-- syso/test.s --
+#include "textflag.h"
+
+TEXT ·Test(SB), NOSPLIT, $0
+	JMP asmTest(SB)
+
+-- syso/test.go --
+package syso
+
+func Test()
+
+-- syso/asm/test.s --
+	.globl asmTest
+asmTest:
+	ret
+
+-- syso/asm/test_darwin.s --
+	.globl _asmTest
+_asmTest:
+	ret
+
+-- cmd/main.go --
+package main
+
+import "syso"
+
+func main() {
+	syso.Test()
+}

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

@@ -111,6 +111,7 @@ func ldpkg(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, filename s
 		}
 		p1 += p0
 
+		lib.Cgo = true
 		loadcgo(ctxt, filename, objabi.PathToPrefix(lib.Pkg), data[p0:p1])
 	}
 }

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

@@ -542,10 +542,10 @@ func (ctxt *Link) loadlib() {
 	}
 	dynexp = dynexp[:w]
 
-	// In internal link mode, read the host object files.
-	if ctxt.LinkMode == LinkInternal {
-		hostobjs(ctxt)
+	// Read the host object files.
+	hostobjs(ctxt)
 
+	if ctxt.LinkMode == LinkInternal {
 		// If we have any undefined symbols in external
 		// objects, try to read them from the libgcc file.
 		any := false
@@ -918,12 +918,13 @@ func loadobjfile(ctxt *Link, lib *sym.Library) {
 }
 
 type Hostobj struct {
-	ld     func(*Link, *bio.Reader, string, int64, string)
-	pkg    string
-	pn     string
-	file   string
-	off    int64
-	length int64
+	ld       func(*Link, *bio.Reader, string, int64, string)
+	pkg      string
+	pn       string
+	file     string
+	off      int64
+	length   int64
+	isCgoPkg bool // part of a Cgo package
 }
 
 var hostobj []Hostobj
@@ -939,7 +940,7 @@ var internalpkg = []string{
 	"runtime/msan",
 }
 
-func ldhostobj(ld func(*Link, *bio.Reader, string, int64, string), headType objabi.HeadType, f *bio.Reader, pkg string, length int64, pn string, file string) *Hostobj {
+func ldhostobj(ld func(*Link, *bio.Reader, string, int64, string), headType objabi.HeadType, f *bio.Reader, lib *sym.Library, pkg string, length int64, pn string, file string) *Hostobj {
 	isinternal := false
 	for _, intpkg := range internalpkg {
 		if pkg == intpkg {
@@ -972,6 +973,7 @@ func ldhostobj(ld func(*Link, *bio.Reader, string, int64, string), headType obja
 	h.file = file
 	h.off = f.Offset()
 	h.length = length
+	h.isCgoPkg = lib.Cgo
 	return h
 }
 
@@ -980,6 +982,13 @@ func hostobjs(ctxt *Link) {
 
 	for i := 0; i < len(hostobj); i++ {
 		h = &hostobj[i]
+		if h.isCgoPkg && ctxt.LinkMode == LinkExternal {
+			// The Hostobj is part of a cgo package during an external link.
+			// Any host objects within this package are accessed from symbols
+			// already loaded from a Go object file, so we do not need to load
+			// any of the host object symbols in this file.
+			continue
+		}
 		f, err := bio.Open(h.file)
 		if err != nil {
 			Exitf("cannot reopen %s: %v", h.pn, err)
@@ -1620,7 +1629,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 			ehdr.flags = flags
 			ctxt.Textp = append(ctxt.Textp, textp...)
 		}
-		return ldhostobj(ldelf, ctxt.HeadType, f, pkg, length, pn, file)
+		return ldhostobj(ldelf, ctxt.HeadType, f, lib, pkg, length, pn, file)
 	}
 
 	if magic&^1 == 0xfeedface || magic&^0x01000000 == 0xcefaedfe {
@@ -1632,7 +1641,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 			}
 			ctxt.Textp = append(ctxt.Textp, textp...)
 		}
-		return ldhostobj(ldmacho, ctxt.HeadType, f, pkg, length, pn, file)
+		return ldhostobj(ldmacho, ctxt.HeadType, f, lib, pkg, length, pn, file)
 	}
 
 	if c1 == 0x4c && c2 == 0x01 || c1 == 0x64 && c2 == 0x86 {
@@ -1647,7 +1656,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 			}
 			ctxt.Textp = append(ctxt.Textp, textp...)
 		}
-		return ldhostobj(ldpe, ctxt.HeadType, f, pkg, length, pn, file)
+		return ldhostobj(ldpe, ctxt.HeadType, f, lib, pkg, length, pn, file)
 	}
 
 	if c1 == 0x01 && (c2 == 0xD7 || c2 == 0xF7) {
@@ -1659,7 +1668,7 @@ func ldobj(ctxt *Link, f *bio.Reader, lib *sym.Library, length int64, pn string,
 			}
 			ctxt.Textp = append(ctxt.Textp, textp...)
 		}
-		return ldhostobj(ldxcoff, ctxt.HeadType, f, pkg, length, pn, file)
+		return ldhostobj(ldxcoff, ctxt.HeadType, f, lib, pkg, length, pn, file)
 	}
 
 	/* check the header */

src/cmd/link/internal/sym/library.go

@@ -17,6 +17,7 @@ type Library struct {
 	DupTextSyms   []*Symbol // dupok text symbols defined in this library
 	Main          bool
 	Safe          bool
+	Cgo           bool
 }
 
 func (l Library) String() string {