CV32E40Pv2 CORE-V SIMD Extension Only

    * gcc/common/config/riscv/riscv-common.cc: Added XCOREVSIMD
      extensions. Added riscv_xcorev_flags to SIMD extensions.
    * gcc/config/riscv/riscv-opts.h: Added corresponding MASK and
      TARGET macros for XCOREVSIMD.