cmd/link: add LIBRARY statement only with -buildmode=cshared

When creating a .def file for Windows linking, add a LIBRARY statement
only when building a DLL with -buildmode=cshared. That statement is
documented to instruct the linker to create a DLL, overriding any
other flag that might indicate building an executable.

Fixes #75734

Change-Id: I0231435df70b71a493a39deb639f6328a8e354f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/708815
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dominic Della Valle <ddvpublic@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: qmuntal

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

@@ -1758,7 +1758,9 @@ func peCreateExportFile(ctxt *Link, libName string) (fname string) {
 	fname = filepath.Join(*flagTmpdir, "export_file.def")
 	var buf bytes.Buffer
 
-	fmt.Fprintf(&buf, "LIBRARY %s\n", libName)
+	if ctxt.BuildMode == BuildModeCShared {
+		fmt.Fprintf(&buf, "LIBRARY %s\n", libName)
+	}
 	buf.WriteString("EXPORTS\n")
 
 	ldr := ctxt.loader