RISC-V: Leverage __builtin_xx instead of math.h for test

RISC-V: Leverage __builtin_xx instead of math.h for test

The math.h may have problems in some environment, take __builtin__xx
instead for testing.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/vls/floating-point-max-5.c:
	Remove reference to math.h.
	* gcc.target/riscv/rvv/autovec/vls/floating-point-min-5.c: Ditto.
	* gcc.target/riscv/rvv/autovec/vls/floating-point-sgnjx-2.c: Ditto.

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