arm: use -march when compiling m5op_arm.S

arm: use -march when compiling m5op_arm.S

Using arm-linux-gnueabi-gcc 4.7.3-1ubuntu1 on Ubuntu 13.04 to compiled
the m5 binary yields the error:

m5op_arm.S: Assembler messages:
m5op_arm.S:85: Error: selected processor does not support ARM mode `bxj lr'

For each of of the SIMPLE_OPs. Apparently, this compiler doesn't like the
interworking of these code types for the default arch. Adding -march=armv7-a
makes it compile. Another alternative that I found to work is replacing the
bxj lr instruction with mov pc, lr, but I don't know how that affects the
KVM stuff and if bxj is needed.