Allow HAVE_DL_ITERATE_PHDR to be defined by client (musl)

Allow HAVE_DL_ITERATE_PHDR to be defined by client (musl)

Do not redefine HAVE_DL_ITERATE_PHDR macro if already defined.

* dyn_load.c [OPENBSD>=200519] (HAVE_DL_ITERATE_PHDR): Do not define
if already defined.
* dyn_load.c [!USE_PROC_FOR_LIBRARIES && (__GLIBC__>=2
|| PLATFORM_ANDROID)] (HAVE_DL_ITERATE_PHDR): Likewise.
* dyn_load.c [!USE_PROC_FOR_LIBRARIES && HAVE_DL_ITERATE_PHDR
&& !DL_ITERATE_PHDR_STRONG] (dl_iterate_phdr): Define as weak symbol
(even if HAVE_DL_ITERATE_PHDR is defined outside this file).