cmd/link: fix plugin on linux/arm64

The init function and runtime.addmoduledata were not added when
building plugin, which caused the runtime could not find the
module.

Testplugin is still not enabled on linux/arm64
(https://go.googlesource.com/go/+/master/src/cmd/dist/test.go#948)
because the gold linker on the builder is too old, which fails
with an internal error (see issue #17138). I tested locally and
it passes.

Fixes #24940.
Updates #17138.

Change-Id: I26aebca6c38a3443af0949471fa12b6d550e8c6c
Reviewed-on: https://go-review.googlesource.com/109917
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/arm64/asm.go

@@ -46,7 +46,7 @@ func gentext(ctxt *ld.Link) {
 		return
 	}
 	addmoduledata := ctxt.Syms.Lookup("runtime.addmoduledata", 0)
-	if addmoduledata.Type == sym.STEXT {
+	if addmoduledata.Type == sym.STEXT && ctxt.BuildMode != ld.BuildModePlugin {
 		// we're linking a module containing the runtime -> no need for
 		// an init function
 		return
@@ -72,7 +72,7 @@ func gentext(ctxt *ld.Link) {
 	rel.Sym = ctxt.Moduledata
 	rel.Type = objabi.R_ADDRARM64
 
-	// 8:	14000000 	bl	0 <runtime.addmoduledata>
+	// 8:	14000000 	b	0 <runtime.addmoduledata>
 	// 	8: R_AARCH64_CALL26	runtime.addmoduledata
 	o(0x14000000)
 	rel = initfunc.AddRel()
@@ -81,6 +81,9 @@ func gentext(ctxt *ld.Link) {
 	rel.Sym = ctxt.Syms.Lookup("runtime.addmoduledata", 0)
 	rel.Type = objabi.R_CALLARM64 // Really should be R_AARCH64_JUMP26 but doesn't seem to make any difference
 
+	if ctxt.BuildMode == ld.BuildModePlugin {
+		ctxt.Textp = append(ctxt.Textp, addmoduledata)
+	}
 	ctxt.Textp = append(ctxt.Textp, initfunc)
 	initarray_entry := ctxt.Syms.Lookup("go.link.addmoduledatainit", 0)
 	initarray_entry.Attr |= sym.AttrReachable