target/arm: Improve IMPDEF algorithm for IRG

When GCR_EL1.RRND==1, the choosing of the random value is IMPDEF,
and the kernel is not expected to have set RGSR_EL1.  Force a
non-zero value into SEED, so that we do not continually return
the same tag.

Reported-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200724163853.504655-4-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>