RISC-V: Support RVV VFDIV and VFRDIV rounding mode intrinsic API

RISC-V: Support RVV VFDIV and VFRDIV rounding mode intrinsic API

This patch would like to support the rounding mode API for the
VFDIV and VFRDIV for the below samples.

* __riscv_vfdiv_vv_f32m1_rm
* __riscv_vfdiv_vv_f32m1_rm_m
* __riscv_vfdiv_vf_f32m1_rm
* __riscv_vfdiv_vf_f32m1_rm_m
* __riscv_vfrdiv_vf_f32m1_rm
* __riscv_vfrdiv_vf_f32m1_rm_m

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

	* config/riscv/riscv-vector-builtins-bases.cc
	(binop_frm): New declaration.
	(reverse_binop_frm): Likewise.
	(BASE): Likewise.
	* config/riscv/riscv-vector-builtins-bases.h:
	(vfdiv_frm): New extern declaration.
	(vfrdiv_frm): Likewise.
	* config/riscv/riscv-vector-builtins-functions.def
	(vfdiv_frm): New function definition.
	(vfrdiv_frm): Likewise.
	* config/riscv/vector.md: Add vfdiv to frm_mode.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/float-point-single-div.c: New test.
	* gcc.target/riscv/rvv/base/float-point-single-rdiv.c: New test.