Lock on dl_interate_phdr

There is possibility of someone dlclosing a library
while dl_iterate_phdr in progress which can lead to
dl_iterate_phdr calling callback with invalid address
if it was unmapped by dlclose.

Bug: http://b/22047255
Change-Id: I3fc0d9fd2c51fb36fd34cb035f37271fa893a7be

linker/linker.cpp

@@ -353,7 +353,7 @@ _Unwind_Ptr dl_unwind_find_exidx(_Unwind_Ptr pc, int* pcount) {
 
 // Here, we only have to provide a callback to iterate across all the
 // loaded libraries. gcc_eh does the rest.
-int dl_iterate_phdr(int (*cb)(dl_phdr_info* info, size_t size, void* data), void* data) {
+int do_dl_iterate_phdr(int (*cb)(dl_phdr_info* info, size_t size, void* data), void* data) {
   int rv = 0;
   for (soinfo* si = solist; si != nullptr; si = si->next) {
     dl_phdr_info dl_info;