cmd/link: delete stale deadcode reference

Back when the linker did code generation after dead code elimination,
it had to know that references to runtime.read_tls_fallback could be
generated at code generation time (and never appear before that). Now
that code generation is done by the compiler, the references to
runtime.read_tls_fallback are obvious in the relocations, so the
linker no longer needs special knowledge of this symbol.

Change-Id: I9813a8478e85a6a13470b2d0528db53fd33fcfdf
Reviewed-on: https://go-review.googlesource.com/c/154601
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -209,11 +209,6 @@ func (d *deadcodepass) markMethod(m methodref) {
 func (d *deadcodepass) init() {
 	var names []string
 
-	if d.ctxt.Arch.Family == sys.ARM {
-		// mark some functions that are only referenced after linker code editing
-		names = append(names, "runtime.read_tls_fallback")
-	}
-
 	if d.ctxt.BuildMode == BuildModeShared {
 		// Mark all symbols defined in this library as reachable when
 		// building a shared library.