target/arm: optimize architecture flags

target/arm: optimize architecture flags

In target/arm.h the struct arm do contain 3 flags to retain architecture
version for some tweaks.
The proposal is to have only one enumerated flag 'arch' for the same purpose.

Change-Id: Ia5d5accfed8158ca21eb54af2fdea8e36f0266ae
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/6229
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>