Userspace ARM BE8 support

Add support for ARM BE8 userspace binaries.
i.e. big-endian data and little-endian code.
In principle LE8 mode is also possible, but AFAIK has never actually
been implemented/used.

System emulation doesn't have any useable big-endian board models,
but should in principle work once you fix that.
Dynamic endianness switching requires messing with data accesses,
preferably with TCG cooperation, and is orthogonal to BE8 support.

Signed-off-by: Paul Brook <paul@codesourcery.com>
[PMM: various changes, mostly as per my suggestions in code review:
 * rebase
 * use EF_ defines rather than hardcoded constants
 * make bswap_code a bool for future VMSTATE macro compatibility
 * update comment in cpu.h about TB flags bit field usage
 * factor out load-code-and-swap into arm_ld*_code functions and
   get_user_code* macros
 * fix stray trailing space at end of line
 * added braces in disas.c to satisfy checkpatch
]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
