scripts/gdb: add cpu iterators

The linux kernel provides macro's for iterating against values from the
cpu_list masks.  By providing some commonly used masks, we can mirror
the kernels helper macros with easy to use generators.

Link: http://lkml.kernel.org/r/d045c6599771ada1999d49612ee30fd2f9acf17f.1462865983.git.jan.kiszka@siemens.com


Signed-off-by: Kieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>