kgdb,arm: add support for thumb2 kernel

Add 16b breakpoint instructions for thumb2 kernel. This repairs the following
error:

  SysRq : DEBUG
  Internal error: Oops - undefined instruction: 0 [#1] SMP THUMB2

...and allows to enter kdb properly under thumb2 kernel for example.

Signed-off-by: Vincent Stehlé <v-stehle@ti.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: kgdb-bugreport@lists.sourceforge.net

TODO
----
- Clarify if we need 16b & 32b as well, like kprobe does.