RISC-V: Add test for FP ifloor auto vectorization

RISC-V: Add test for FP ifloor auto vectorization

The below FP API are supported already by sharing the same standard
name, as well as the machine mode.

int ifloor (float);

This patch would like to add the test cases for ensuring the
correctness.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/math-ifloor-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-ifloor-run-0.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-ifloor-0.c: New test.

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