cmd/compile: fix arm direct tail call

CL 752560 accidentally niled out the target symbol.
Re-initialize the target symbol field when doing a direct tail call.

Fixes #78053

Change-Id: I007471587a38077b52daa134640d09c7b8b6becb
Reviewed-on: https://go-review.googlesource.com/c/go/+/753840
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/internal/obj/arm/obj5.go

@@ -367,6 +367,8 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 					p.From = obj.Addr{}
 					if retSym != nil { // retjmp
 						p.To.Type = obj.TYPE_BRANCH
+						p.To.Name = obj.NAME_EXTERN
+						p.To.Sym = retSym
 					} else {
 						p.To.Type = obj.TYPE_MEM
 						p.To.Offset = 0