[RISCV] Rename FeatureRVE to FeatureStdExtE. NFC (#89174)

Planning to declare all extensions in tablegen so we can generate the
tables for RISCVISAInfo.cpp. This requires making "e" consistent with
other extensions.

GitOrigin-RevId: 016ce9ed5cd3694cbff72a768a593714913822ea
6 files changed