target/arm: implement FEAT_RNG_TRAP for RNDR/RNDRRS

Add an .accessfn to the RNDR and RNDRRS system registers that traps
reads to EL3 when SCR_EL3.TRNDR is set, as required by FEAT_RNG_TRAP.
Mark SCR_EL3.TRNDR (bit 40) as a writable field in scr_write() when
the CPU advertises the feature. The pseudocode in DDI0487 revision M.b
shows the trap firing from EL0, EL1, EL2, and EL3, so there is no
check of arm_current_el().

When FEAT_RNG_TRAP is implemented without FEAT_RNG, an RNDR/RNDRRS read
with SCR_EL3.TRNDR=0 should UNDEF rather than succeed; handle that case
in access_rndr(). Register the rndr_reginfo CP reg entries whenever either
FEAT_RNG or FEAT_RNG_TRAP is implemented, so the accessfn fires even on a
FEAT_RNG_TRAP-only CPU.

When SCR_EL3.TRNDR is set, ID_AA64ISAR0_EL1.RNDR reads as 1 regardless
of whether FEAT_RNG is implemented; give ID_AA64ISAR0_EL1 a readfn so it
reports this at runtime, as we already do for ID_AA64PFR0_EL1.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Jason Wright <wrigjl@proton.me>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>