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 +1,47 @@
+# Test that we can link a syso file that is embedded in a package, that is
+# referenced by an assembly stub.  See issue 33139.
+#
+# We use cgo only to enforce external linkage of the main executable.
+[!cgo] stop
+[!gc] stop
+[linux] cc -c -o syso/test.syso syso/asm/test_linux.s
+[darwin] cc -c -o syso/test.syso syso/asm/test_darwin.s
+go build ./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_linux.s --
+	.globl asmTest
+asmTest:
+	ret
+
+-- syso/asm/test_darwin.s --
+	.globl _asmTest
+_asmTest:
+	ret
+
+-- cmd/main.go --
+package main
+
+/*
+ // Force use of external linker
+ #include <stdio.h>
+ */
+import "C"
+
+import (
+	"syso"
+)
+
+func main() {
+	syso.Test()
+}

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

@@ -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
+	internal bool
 }
 
 var hostobj []Hostobj
@@ -972,14 +973,23 @@ func ldhostobj(ld func(*Link, *bio.Reader, string, int64, string), headType obja
 	h.file = file
 	h.off = f.Offset()
 	h.length = length
+	h.internal = isinternal
 	return h
 }
 
 func hostobjs(ctxt *Link) {
+	// load all hostobj
+	hostobjsExternalOnly(ctxt, false)
+}
+
+func hostobjsExternalOnly(ctxt *Link, sel bool) {
 	var h *Hostobj
 
 	for i := 0; i < len(hostobj); i++ {
 		h = &hostobj[i]
+		if sel && h.internal {
+			continue
+		}
 		f, err := bio.Open(h.file)
 		if err != nil {
 			Exitf("cannot reopen %s: %v", h.pn, err)
@@ -1002,6 +1012,10 @@ func hostlinksetup(ctxt *Link) {
 	debug_s = *FlagS
 	*FlagS = false
 
+	// load non-internal host objects, populating the symbol table so that
+	// reloc can work
+	hostobjsExternalOnly(ctxt, true)
+
 	// create temporary directory and arrange cleanup
 	if *flagTmpdir == "" {
 		dir, err := ioutil.TempDir("", "go-link-")