target/cortex_m: avoid dwt comparator overflow

target/cortex_m: avoid dwt comparator overflow

Avoid ever overflowing the DWT_COMPARATOR array by allocating space for
16 comparators (the field is masked by 0xf).

On a stm32f767zi chip (on a nucleo-767zi board) I've been seeing crashes
with address sanitizer enabled due to its (apparent) 10 present
comparators. This appears to be due to
https://sourceforge.net/p/openocd/tickets/178/.

In non-address sanitizer builds, this would likely cause some random
memory to be written to in some cases. (see above bug for observations).

Change-Id: I2b7d599eb326236dbc93f74b350c442c9a502c4b
Signed-off-by: Cody P Schafer <openocd@codyps.com>
Reviewed-on: http://openocd.zylin.com/4458
Tested-by: jenkins
Reviewed-by: Christopher Head <chead@zaber.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>