[RISCV] Remove 'xcv' extension

Created by: CharKeaney

The CORE-V ISA Extension Naming specification was recently updated. One of the consequences of this is that the option previously called 'xcorev' (but implemented as 'xcv' presently due to the prefix change) has been removed. However, the compiler does not presently reflect this change. This patch removes the 'xcv' march flag option so that the compiler and the CORE-V ISA Extension Naming specification match.

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