ARM: use -mcpu=cortex-a9 when targeting U8420 CPUs (Cortex-A9 based)
This is done in order to force compliant GCC compiler to generate the new
sdiv/udiv instructions supported by Cortex-A9 architecture

Change-Id: I6b27e5eeadad17259f77aac53fabfcaf257657fe
Signed-off-by: zwliew <zhaoweiliew@gmail.com>