[RISCV] Mark zvkn* and zvks* extensions as enabled when all their subextensions are.

These extensions don't contain any instructions on their own, they
are just aliases for a set of extensions. We should set the preprocessor
define anytime all the sub-extensions are supported.

Reviewed By: kito-cheng, eopXD

Differential Revision: https://reviews.llvm.org/D154171