tcg/arm: Fix memory barrier encoding
    
    I found with qemu 2.11.x or newer that I would get an illegal instruction
    error running some Intel binaries on my ARM chromebook.  On investigation,
    I found it was quitting on memory barriers.
    
    qemu instruction:
    mb $0x31
    was translating as:
    0x604050cc:  5bf07ff5  blpl     #0x600250a8
    
    After patch it gives:
    0x604050cc:  f57ff05b  dmb      ish
    
    In short, I found INSN_DMB_ISH (memory barrier for ARMv7) appeared to be
    correct based on online docs, but due to some endian-related shenanigans it
    had to be byte-swapped to suit qemu; it appears INSN_DMB_MCR (memory
    barrier for ARMv6) also should be byte swapped  (and this patch does so).
    I have not checked for correctness of aarch64's barrier instruction.
    
    Cc: qemu-stable@nongnu.org
    Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
    Signed-off-by: Henry Wertz <hwertz10@gmail.com>
    Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
    (cherry picked from commit 3f814b803797c007abfe5c4041de754e01723031)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>