RISC-V: Refine run test cases of math autovec

RISC-V: Refine run test cases of math autovec

For the run test cases of math autovec, we need a reference value to
check if the return value is expected or not.

The previous patch leverage hardcode for the reference value but we
can leverage the scalar math function instead. For example ceil after
autovec.

ASSERT (CEIL (Vector {1.2,...}) == Vector {2.0, ...});

But we can leverage the scalar math function to avoid potential mistakes.

ASSERT (CEIL (Vector {1.2,...}) == Vector {ceil (1.2), ...});

This patch remove some fflags check as it covered by check-body already.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/math-ceil-run-1.c:
	Use scalar func as reference instead of hardcode.
	* gcc.target/riscv/rvv/autovec/unop/math-ceil-run-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-floor-run-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-floor-run-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-nearbyint-run-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-nearbyint-run-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-rint-run-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-rint-run-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-round-run-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-round-run-2.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-trunc-run-1.c: Ditto.
	* gcc.target/riscv/rvv/autovec/unop/math-trunc-run-2.c: Ditto.

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