RISC-V: Add support to vector stack-clash protection

Adds basic support to vector stack-clash protection using a loop to do
the probing and stack adjustments.

gcc/ChangeLog:
	* config/riscv/riscv.cc
	(riscv_allocate_and_probe_stack_loop): New function.
	(riscv_v_adjust_scalable_frame): Add stack-clash protection
	support.
	(riscv_allocate_and_probe_stack_space): Move the probe loop
	implementation to riscv_allocate_and_probe_stack_loop.
	* config/riscv/riscv.h: Define RISCV_STACK_CLASH_VECTOR_CFA_REGNUM.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/stack-check-cfa-3.c: New test.
	* gcc.target/riscv/stack-check-prologue-16.c: New test.
	* gcc.target/riscv/struct_vect_24.c: New test.