arch-arm: Add basic support for KVM_CAP_ARM_USER_IRQ

arch-arm: Add basic support for KVM_CAP_ARM_USER_IRQ

KVM_CAP_ARM_USER_IRQ is a KVM extension introduced in newer versions of
Linux (>= 4.12). It supports delivering interrupt from the kernel-space
timer to the user-space GIC, which means that it will be unnecessary to
use the memory-mapped timer and emulate it in gem5 anymore.

Using the option provided by this change, Linux is able to boot with 1
CPU successfully, and the speed is slightly faster then the memory-
mapped timer option. However, multicore seems to hang during boot and
still needs more investigation to be enabled.

JIRA: https://gem5.atlassian.net/browse/GEM5-663

Change-Id: I146bbcce3cf66f8f5ebee04ea5f1b9f54868721a
Signed-off-by: Hsuan Hsu <hsuan.hsu@mediatek.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30921
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>