target-arm: introduce disas flag for endianness

target-arm: introduce disas flag for endianness

Introduce a disas flag for setting the CPU data endianness. This allows
control of the endianness from the CPU state rather than hard-coding it
to TARGET_WORDS_BIGENDIAN.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[ PC changes:
  * Split off as new patch from original:
        "target-arm: introduce tbflag for CPSR.E"
  * Wrote commit message from scratch
]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>