RISC-V: Reconcile the existing test for avg_ceil

RISC-V: Reconcile the existing test for avg_ceil

Some existing avg_floor test need updated due to change to
leverage vaadd.vv directly.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/avg-4.c: Update asm check
	to vaadd.
	* gcc.target/riscv/rvv/autovec/vls/avg-5.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/avg-6.c: Ditto.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv32gcv.c: Ditto.
	* gcc.target/riscv/rvv/autovec/widen/vec-avg-rv64gcv.c: Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>