Fix dyn_load.c compilation for Android 4.3

* dyn_load.c (struct link_map, r_debug): Define explicitly (matching
that of Android 4.2 Bionic) instead of include linker.h (only if
PLATFORM_ANDROID); remove workaround enabled by
BIONIC_ELFDATA_REDEF_BUG regarding ELF_DATA redefinition (as linker.h
is not included); update comment.