Order GC_register_dynamic_libraries definitions lexicographically

(refactoring)

* dyn_load.c [OSF1]: Do not check if `ALPHA` is defined.
* dyn_load.c [AIX || DARWIN || HAIKU || HPUX]
(GC_register_dynamic_libraries): Order function definition
lexicographically by platform.
* dyn_load.c [USE_PROC_FOR_LIBRARIES && !LINUX || IRIX5]: Place include
`signal.h` nearby ifndef `_sigargs`.
* dyn_load.c [DYNAMIC_LOADING && !ANY_MSWIN && !(CPPCHECK || AIX
|| DARWIN || DGUX || IRIX5 || HAIKU || HPUX || HURD || NACL || OSF1
|| SCO_ELF || SOLARISDL || (ANY_BSD || LINUX) && __ELF__ || OPENBSD
&& M68K)]: Shorten message of preprocessor `error` directive.
* dyn_load.c [DYNAMIC_LOADING] (GC_register_main_static_data): Remove
duplicate comments.
* dyn_load.c [DYNAMIC_LOADING] (HAVE_REGISTER_MAIN_STATIC_DATA): Move
be right after `GC_register_main_static_data` definition.
* dyn_load.c [DYNAMIC_LOADING && !ANY_MSWIN && (DGUX || HURD || NACL
|| SCO_ELF || (ANY_BSD || LINUX) && __ELF__)
&& USE_PROC_FOR_LIBRARIES]: Order include directives lexicographically.
* dyn_load.c [DYNAMIC_LOADING && !ANY_MSWIN && (DGUX || HURD || NACL
|| SCO_ELF || (ANY_BSD || LINUX) && __ELF__)
&& !USE_PROC_FOR_LIBRARIES && !HAVE_DL_ITERATE_PHDR]: Reformat comment.
* dyn_load.c [DYNAMIC_LOADING && HPUX && THREADS]
(GC_register_dynamic_libraries): Remove duplicate check of `HPUX`.