target-arm: Switch ARMCPUInfo arrays to use terminator entries

Switch the ARMCPUInfo arrays in cpu.c and cpu64.c to use a terminator
entry rather than looping based on ARRAY_SIZE. The latter causes
compile warnings on some versions of gcc if the configure options
happen to result in an empty array.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
