RISC-V: Support vfadd static rounding mode by mode switching

RISC-V: Support vfadd static rounding mode by mode switching

This patch would like to support the vfadd static round mode similar to
the fixed-point. Then the related fsrm instructions will be inserted
correlatively.

Please *NOTE* this PATCH doesn't cover anything about FRM dynamic mode,
it will be implemented in the underlying PATCH(s).

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

	* config/riscv/riscv.cc (riscv_emit_mode_set): Add emit for FRM.
	(riscv_mode_needed): Likewise.
	(riscv_entity_mode_after): Likewise.
	(riscv_mode_after): Likewise.
	(riscv_mode_entry): Likewise.
	(riscv_mode_exit): Likewise.
	* config/riscv/riscv.h (NUM_MODES_FOR_MODE_SWITCHING): Add number
	for FRM.
	* config/riscv/riscv.md: Add FRM register.
	* config/riscv/vector-iterators.md: Add FRM type.
	* config/riscv/vector.md (frm_mode): Define new attr for FRM mode.
	(fsrm): Define new insn for fsrm instruction.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/float-point-frm-insert-1.c: New test.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-2.c: New test.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-3.c: New test.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-4.c: New test.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-5.c: New test.