RISC-V: Support FP llrint auto vectorization

This patch would like to support the FP llrint auto vectorization.

* long long llrint (double)

This will be the CVT from DF => DI from the standard name's perpsective,
which has been covered in previous PATCH(es). Thus, this patch only add
some test cases.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/unop/test-math.h: Add type int64_t.
	* gcc.target/riscv/rvv/autovec/unop/math-llrint-0.c: New test.
	* gcc.target/riscv/rvv/autovec/unop/math-llrint-run-0.c: New test.
	* gcc.target/riscv/rvv/autovec/vls/math-llrint-0.c: New test.

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