arch: tile: kernel: kgdb.c: Use memcpy() instead of pointer copy one by one

Not only memcpy() is faster than pointer copy, but also let code more
clearer and simple, which can avoid compiling warning (the original
implementation copy registers by exceeding member array border).

The related warning (with allmodconfig under tile):

    CC      arch/tile/kernel/kgdb.o
  arch/tile/kernel/kgdb.c: In function 'sleeping_thread_to_gdb_regs':
  arch/tile/kernel/kgdb.c:140:31: warning: iteration 53u invokes undefined behavior [-Waggressive-loop-optimizations]
     *(ptr++) = thread_regs->regs[reg];
                                 ^
  arch/tile/kernel/kgdb.c:139:2: note: containing loop
    for (reg = 0; reg <= TREG_LAST_GPR; reg++)
    ^

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>