[MIPS] Rewrite MIPS crtbegin* as C files.

This updates the MIPS arch to be much more in
sync with the commit Nick Kralevich made last
June; see 9d403268.

    Rewrite
     crtbegin.S        -> crtbegin.c
     crtbegin_so.S     -> crtbegin_so.c
     __dso_handle.S    -> __dso_handle.c
     __dso_handle_so.S -> __dso_handle_so.c
     atexit.S          -> atexit.c

Previously __do_global_dtors_aux was in the tasks
__FINI_ARRAY__ linked with crtbegin.S and it now being
removed as there is no need to call a destructor just
before terminating a process.

Shared libraries, on the other hand, are linked with
crtbegin_so.c and have a hidden destructor declared
to allow the bionic linker to call __on_dlclose().

Change-Id: Ieb4da5199b54573de05743990e309db381a11cb8
Signed-off-by: Pete Delaney <piet.delaney@imgtec.com>
Signed-off-by: Chao-Ying Fu <chao-ying.fu@imgtec.com>
Signed-off-by: Chris Dearman <chris.dearman@imgtec.com>