[RISCV] 'K'-extension ordering

This commit adds 'K' to supported extension list (before 'J').
It makes "Zk*" extensions correctly placed before "Zv*" extensions.

Multi-letter "Z*" extensions are first ordered with the most closely
related alphabetical extension category ("IMAF...").  This is represented
in LLVM as `AllStdExts' variable in `llvm/lib/Support/RISCVISAInfo.cpp'.

However, it did not have 'k' making "Zk*" extensions not correctly ordered.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D124340