KVM: selftests: Add array order helpers to riscv get-reg-list

Add a couple macros to use when filling arrays in order to ensure
the elements are placed in the right order, regardless of the
order we prefer to read them. And immediately apply the new
macro to resorting the ISA extension lists alphabetically.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Haibo Xu <haibo1.xu@intel.com>
Signed-off-by: Anup Patel <anup@brainfault.org>