RISC-V: Implement __riscv_vlenb PR109228

__riscv_vlenb is defined in RVV intrinsic spec 0.11 and used in some project
like google/highway.

gcc/ChangeLog:

	PR target/109228
	* config/riscv/riscv-vector-builtins-bases.cc (class vlenb): Add
	__riscv_vlenb support.
	(BASE): Ditto.
	* config/riscv/riscv-vector-builtins-bases.h: Ditto.
	* config/riscv/riscv-vector-builtins-functions.def (vlenb): Ditto.
	* config/riscv/riscv-vector-builtins-shapes.cc (struct vlenb_def): Ditto.
	(SHAPE): Ditto.
	* config/riscv/riscv-vector-builtins-shapes.h: Ditto.
	* config/riscv/riscv-vector-builtins.cc: Ditto.

gcc/testsuite/ChangeLog:

	PR target/109228
	* gcc.target/riscv/rvv/base/vlenb-1.c: New test.