Listing the set of host architectures does not scale.
Depend instead on the existence of the architecture rng.

This will allow RANDOM_TRUST_CPU to be selected on arm64. Today
ARCH_RANDOM is only selected by x86, s390, and powerpc, so this does not
adversely affect other architectures.
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20200210130015.17664-5-mark.rutland@arm.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>