runtime: fix TODO comment filepath

The cmd/compile/internal/ld/go.go file not exist, actually cmd/link/internal/ld/go.go.
Also, write line number is not good because it changes every commit of the file.

Change-Id: Id2b9f2c9904390adb011dab357716ee8e2fe84fc
Reviewed-on: https://go-review.googlesource.com/135516
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: zchee

src/runtime/sys_darwin.go

@@ -370,5 +370,5 @@ func closeonexec(fd int32) {
 //go:cgo_import_dynamic libc_pthread_cond_signal pthread_cond_signal "/usr/lib/libSystem.B.dylib"
 
 // Magic incantation to get libSystem actually dynamically linked.
-// TODO: Why does the code require this?  See cmd/compile/internal/ld/go.go:210
+// TODO: Why does the code require this?  See cmd/link/internal/ld/go.go
 //go:cgo_import_dynamic _ _ "/usr/lib/libSystem.B.dylib"