Add platform specific version tags.

Some functions were added at different times on different platforms.
This should hopefully get most of them.

Bug: http://b/28178111
Change-Id: I85fdf92779a2f3277e9b537c6bfc7216790c607a
diff --git a/libc/include/link.h b/libc/include/link.h
index e6c5404..2d189f6 100644
--- a/libc/include/link.h
+++ b/libc/include/link.h
@@ -46,7 +46,11 @@
   ElfW(Half) dlpi_phnum;
 };
 
+#if defined(__arm__)
 int dl_iterate_phdr(int (*)(struct dl_phdr_info*, size_t, void*), void*) __INTRODUCED_IN(21);
+#else
+int dl_iterate_phdr(int (*)(struct dl_phdr_info*, size_t, void*), void*);
+#endif
 
 #ifdef __arm__
 typedef long unsigned int* _Unwind_Ptr;