[RISCV] Fix RISCVTargetInfo::initFeatureMap, add non-ISA features back after implication

Previously D113336 makes RISCVTargetInfo::initFeatureMap return the results
processed by RISCVISAInfo, which only consists of ISA features and misses
non-ISA features like `relax` and `save-restore`.

This patch fixes the problem.

Reviewed By: junparser

Differential Revision: https://reviews.llvm.org/D119541