RISC-V: Elimilate warning in class vcreate

RISC-V: Elimilate warning in class vcreate

The following is the content of class vcreate:
class vcreate : public function_base
{
public:
  gimple *fold (gimple_folder &f) const override
  {
    ....
  }

  rtx expand (function_expander &e) const override
  {
    return NULL_RTX;
  }
};

The warning caused is:
./riscv-gcc/gcc/config/riscv/riscv-vector-builtins-bases.cc:1719:34:
  warning: unused parameter 'e' [-Wunused-parameter]
  rtx expand (function_expander &e) const override
                                 ^

gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: remove unused
		parameter e and replace NULL_RTX with gcc_unreachable.