RISC-V: ADJUST_NUNITS according to -march.

This patch fixed PR107357: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107357

gcc/ChangeLog:

	PR target/107357
	* config/riscv/riscv-modes.def (VECTOR_BOOL_MODE): Set to minimum size.
	(ADJUST_NUNITS): Adjust according to -march.
	(ADJUST_BYTESIZE): Ditto.
	* config/riscv/riscv-protos.h (riscv_v_ext_enabled_vector_mode_p):
	Remove.
	(riscv_v_ext_vector_mode_p): Change function implementation.
	* config/riscv/riscv-vector-builtins.cc (rvv_switcher::rvv_switcher):
	Change to riscv_v_ext_vector_mode_p.
	(register_builtin_type): Ditto.
	* config/riscv/riscv.cc (riscv_v_ext_vector_mode_p): Change to enabled
	modes.
	(ENTRY): Ditto.
	(riscv_v_ext_enabled_vector_mode_p): Remove.
	(riscv_v_adjust_nunits): New function.
	(riscv_vector_mode_supported_p): Use riscv_v_ext_vector_mode_p instead.
	* config/riscv/riscv.h (riscv_v_adjust_nunits): New function.