[RISCV] Update CORE-V Extension name prefix from 'xcorev' to 'xcv'

Created by: CharKeaney

The CORE-V ISA Extension Naming specification was recently updated. One of the changes was to change the CORE-V Extension name prefix from 'xcorev' to 'xcv'. LLVM currently prefixes corev extensions with the old prefix 'xcorev' rather than the new prefix 'xcv'. LLVM must be updated in order to reflect this change. This patch changes all references to 'xcorev' with 'xcv'. This also includes references in the test suite. It also replaces internal variables that referred to the extensions as COREV_[EXTENSION_NAME] to CV_[EXTENSION_NAME].

See here for CORE-V ISA Extension Naming specification: (https://github.com/openhwgroup/core-v-sw/blob/master/specifications/corev-isa-extension-naming.md)