RISC-V: Always define MULTILIB_DEFAULTS

RISC-V: Always define MULTILIB_DEFAULTS

 - Define MULTILIB_DEFAULTS can reduce the total number of multilib if
   the default arch and ABI are listed in the multilib config.

 - This also simplify the implementation of --with-multilib-list.

gcc/ChangeLog:

	* config.gcc (riscv*-*-*): Add TARGET_RISCV_DEFAULT_ABI and
	TARGET_RISCV_DEFAULT_ARCH to tm_defines.
	Remove including riscv/withmultilib.h for --with-multilib-list.
	* config/riscv/riscv.h (STRINGIZING): New.
	(__STRINGIZING): Ditto.
	(MULTILIB_DEFAULTS): Ditto.
	* config/riscv/withmultilib.h: Remove.