Remove 'xcv' extension

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)

	* gcc/common/config/riscv/riscv-common.cc: Remove 'xcv' extension.
	* gcc/config/riscv/riscv-opts.h: Likewise.