Updated assembler instructions in core_cm3.c 

... to resolve issue with old library vs new gcc.

When using gcc version 4.8.2 to make, the following error was returned: 
/tmp/ccn4Pg52.s: Assembler messages:
/tmp/ccn4Pg52.s:488: Error: registers may not be the same -- `strexb r0,r0,[r1]'
/tmp/ccn4Pg52.s:512: Error: registers may not be the same -- `strexh r0,r0,[r1]'
make[1]: *** [core_cm3.o] Error 1

The solution was found here: http://www.atollic.com/index.php/kb/1-kb_building/117-kb_error_in_strexb