RISC-V: Add TARGET_MIN_VLEN > 4096 check

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_option_override): Add sorry check.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/zvl-unimplemented-1.c: New test.
	* gcc.target/riscv/rvv/base/zvl-unimplemented-2.c: New test.