gdb: Potential rounding error in reg_packet_size gdb_get_registers_packet

gdb: Potential rounding error in reg_packet_size gdb_get_registers_packet

The calculation for reg_packet_size in gdb_get_registers_packet() could
generate a wrong result in the case of multiple registers whose size is
not a multiple of 8.

The current calculation sums the sizes for all registers then rounds the result
up to the next multiple of 8.

Instead it should round each register size up individually and sum the results for all registers.

Change-Id: Idfb5e5eeee0e69a6889dbe9769c0bf17feacb63b
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/200
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>