RISC-V: Fix compilation failed for frflags builtin in C++ mode

RISC-V: Fix compilation failed for frflags builtin in C++ mode

  - g++ will complain too few arguments for frflags builtin like bellow
    message:

    error: too few arguments to function 'unsigned int __builtin_riscv_frflags(void)'

  - However it's no arguments needed, it because we declare the function
    type with VOID arguments, that seems like require a VOID argument
    in the c++ front-end when GCC tried to resolve the function.

gcc/ChangeLog

	* config/riscv/riscv-builtins.c (RISCV_FTYPE_NAME0): New.
	(RISCV_FTYPE_ATYPES0): New.
	(riscv_builtins): Using RISCV_USI_FTYPE for frflags.
	* config/riscv/riscv-ftypes.def: Remove VOID argument.

gcc/testsuite/ChangeLog

	* g++.target/riscv/frflags.C: New.