Add new optimized strlen for arm.

This optimized version is primarily targeted at cortex-a15.

Tested on all nexus devices using the system/extras/libc_test strlen test.
Tested alignments from 1 to 32 that are powers of 2.
Tested that strlen does not cross page boundaries at all alignments.

Speed improvements listed below:

cortex-a15
- Sizes >= 32 bytes, ~75% improvement.
- Sizes >= 1024 bytes, ~250% improvement.

cortex-a9
- Sizes >= 32 bytes, ~75% improvement.
- Sizes >= 1024 bytes, ~85% improvement.

krait
- Sizes >= 32 bytes, ~95% improvement.
- Sizes >= 1024 bytes, ~160% improvement.

Merge from internal master.

(cherry-picked from 2fc071797743b88a9a47427d46baed7c7b24f4d2)

Change-Id: I1ceceb4e745fd68e9d946f96d1d42e0cdaff6ccf
[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 9d40326830c2bd407427889c554adeb915ee6b4a.

    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>
Remove some non-unused makefile generality.

MIPS uses .c files like ARM and x86 now.

Change-Id: Ie580647e385121c380316c443ec199dabf657ff8