target-arm: Implement AArch64 view of CONTEXTIDR

target-arm: Implement AArch64 view of CONTEXTIDR

Implement AArch64 view of the CONTEXTIDR register.
We tighten up the condition when we flush the TLB on a CONTEXTIDR
write to avoid needlessly flushing the TLB every time on a 64
bit system (and also on a 32 bit system using LPAE, as a bonus).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>