Retire the Core_A folder and use a single Core folder for all processor core headers

* Separate folders for different architecture profiles (A-profile, R-profile and M-profile)
* Top-level compiler headers will exist in the Core folder (no separate Compiler folder is needed).
** The cmsis_compiler.h header will continue to figure out which compiler toolchain is being used.
** Compiler headers specific to each architecture profiles can reside within the new architecture profile folders. 
** The second-level files like cmsis_gcc.h are named cmsis_gcc_a.h, cmsis_gcc_r.h and cmsis_gcc_m.h to avoid having multiple files with the same name