rtos/hwthread: fix register list for armv7a

rtos/hwthread: fix register list for armv7a

The targets armv7a in file cortex_a.c inherit the register list
from file armv4_5.c thus, depending on the core status, some
register get marked as not existing.
For HW threads other than current target, the registers in the
list are not checked for existence and are all forwarded to GDB
that in turns complains for too many data:
	  Remote 'g' packet reply is too long (expected 68 bytes, got 104 bytes)

Check all the attributes of the registers and pass to GDB only the
valid registers.

To test it, use a SMP cortex-a target (2 cores are enough) and add
	-rtos hwthread
to all the cores. Connect GDB to OpenOCD and issue the GDB command
	info threads

Change-Id: Ie66119fe83a3c8d53e9d18dda39e60fd97769669
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5981
Tested-by: jenkins