RISC-V: Enable TARGET_SUPPORTS_WIDE_INT

RISC-V: Enable TARGET_SUPPORTS_WIDE_INT

This is at par with other major arches such as aarch64, i386, s390 ...

gcc/ChangeLog

	* config/riscv/predicates.md (const_0_operand): Remove
	const_double.
	* config/riscv/riscv.cc (riscv_rtx_costs): Add check for
	CONST_DOUBLE.
	* config/riscv/riscv.h (TARGET_SUPPORTS_WIDE_INT): New define.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>