arm64: Kconfig: Make CPU_BIG_ENDIAN depend on BROKEN

Big-endian arm64 configurations are vanishingly rare, yet we still claim
to support them in Linux despite very limited testing or visible
interest. Supporting big-endian adds unnecessary burden to reviewers and
contributors which, without any known active users, is hard to justify.
For example, recent work to improve our futex routines and to implement
nested virtualisation support is non-trivially complicated by having to
support both big- and little-endianness.

Back in 2019 [1], it was claimed that Huawei were using arm64 big-endian
machines in their telecommunication products but I don't know whether
that's still the case and certainly haven't seen any patch contributions
to help support or maintain it.

Make CPU_BIG_ENDIAN depend on BROKEN as an initial deprecation step
towards its removal.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Hanjun Guo <guohanjun@huawei.com>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/linux-arm-kernel/73701e9f-bee1-7ae8-2277-7a3576171cd4@huawei.com/

 [1]
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>