linux-user: Add support for big-endian aarch64

Enable big-endian mode for data accesses on aarch64 for big-endian linux
user mode. Activate it for all exception levels as documented by ARM:
Set the SCTLR EE bit for ELs 1 through 3. Additionally set bit E0E in
EL1 to enable it in EL0 as well.

Signed-off-by: Michael Weiser <michael.weiser@gmx.de>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20171220212308.12614-2-michael.weiser@gmx.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>