Add a log message to make the transition from linker to user code clear.

Handy when looking at strace output for seeing "where does my code start?",
though you'll need to set LD_DEBUG=1 (or greater) and constructors will
already have run (but you can set LD_DEBUG to a higher value to track that).

Change-Id: I82d53a7123b39febf5b2a956de83c8a30f229651

linker/linker.cpp

@@ -2675,6 +2675,8 @@ extern "C" ElfW(Addr) __linker_init(void* raw_args) {
 
   protect_data(PROT_READ);
 
+  INFO("[ jumping to _start ]");
+
   // Return the address that the calling assembly stub should jump to.
   return start_address;
 }