target/xtensa: enable all coprocessors for linux-user

Linux userspace always sees coprocessors as enabled. CPENABLE register
and coprocessor exceptions are used internally by the kernel to manage
lazy coprocessor context switch. None of it is needed for linux-user.
Always enable all coprocessors for user emulation.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200829104758.22337-1-jcmvbkbc@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>