RISC-V: Support _Float16 type.

RISC-V: Support _Float16 type.

RISC-V decide use _Float16 as primary IEEE half precision type, and this
already become part of psABI, this patch has added folloing support for
_Float16:

- Soft-float support for _Float16.
- Make sure _Float16 available on C++ mode.
- Name mangling for _Float16 on C++ mode.

gcc/ChangeLog

	* config/riscv/riscv-builtins.cc: include stringpool.h
	(riscv_float16_type_node): New.
	(riscv_init_builtin_types): Ditto.
	(riscv_init_builtins): Call riscv_init_builtin_types.
	* config/riscv/riscv-modes.def (HF): New.
	* config/riscv/riscv.cc (riscv_output_move): Handle HFmode.
	(riscv_mangle_type): New.
	(riscv_scalar_mode_supported_p): Ditto.
	(riscv_libgcc_floating_mode_supported_p): Ditto.
	(riscv_excess_precision): Ditto.
	(riscv_floatn_mode): Ditto.
	(riscv_init_libfuncs): Ditto.
	(TARGET_MANGLE_TYPE): Ditto.
	(TARGET_SCALAR_MODE_SUPPORTED_P): Ditto.
	(TARGET_LIBGCC_FLOATING_MODE_SUPPORTED_P): Ditto.
	(TARGET_INIT_LIBFUNCS): Ditto.
	(TARGET_C_EXCESS_PRECISION): Ditto.
	(TARGET_FLOATN_MODE): Ditto.
	* config/riscv/riscv.md (mode): Add HF.
	(softload): Add HF.
	(softstore): Ditto.
	(fmt): Ditto.
	(UNITMODE): Ditto.
	(movhf): New.
	(*movhf_softfloat): New.

libgcc/ChangeLog:

	* config/riscv/sfp-machine.h (_FP_NANFRAC_H): New.
	(_FP_NANFRAC_H): Ditto.
	(_FP_NANSIGN_H): Ditto.
	* config/riscv/t-softfp32 (softfp_extensions): Add HF related
	routines.
	(softfp_truncations): Ditto.
	(softfp_extras): Ditto.
	* config/riscv/t-softfp64 (softfp_extras): Add HF related routines.

gcc/testsuite/ChangeLog:

	* g++.target/riscv/_Float16.C: New.
	* gcc.target/riscv/_Float16-soft-1.c: Ditto.
	* gcc.target/riscv/_Float16-soft-2.c: Ditto.
	* gcc.target/riscv/_Float16-soft-3.c: Ditto.
	* gcc.target/riscv/_Float16-soft-4.c: Ditto.
	* gcc.target/riscv/_Float16.c: Ditto.