RISC-V: KVM: Make ISA ext mappings explicit

While adding new extensions at the bottom of the array isn't hard to
do, it's a pain to review in order to ensure we're not missing any.
Also, resolving merge conflicts for multiple new ISA extensions can be
error-prone. To make adding new mappings foolproof, explicitly assign
the array elements. And, now that the order doesn't matter, we can
alphabetize the extensions, so we do that too.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Anup Patel <anup@brainfault.org>