Blackfin arch: fix some bugs in lib/string.h functions found by our string testing...

Blackfin arch: fix some bugs in lib/string.h functions found by our string testing modules

 - use ints for the return value rather than char since we actually return
   an int and we dont want it improperly being sign extended during the reload
   http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3525

 - if src is shorter than the requested number of copy bytes, we need to null
   pad the rest
   http://blackfin.uclinux.org/gf/project/uclinux-dist/tracker/?action=TrackerItemEdit&tracker_item_id=3524

 - mark these as __volatile__ and add memory to the clobber list so gcc does
   not optimize buffers around on us we may be using

 - rewrite asm code to be readable/maintainable

Signed-off-by: Mike Frysinger <michael.frysinger@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>