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.

gcc/config/riscv/riscv-builtins.c

@@ -38,6 +38,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "langhooks.h"
 
 /* Macros to create an enumeration identifier for a function prototype.  */
+#define RISCV_FTYPE_NAME0(A) RISCV_##A##_FTYPE
 #define RISCV_FTYPE_NAME1(A, B) RISCV_##A##_FTYPE_##B
 
 /* Classifies the prototype of a built-in function.  */
@@ -121,11 +122,13 @@ AVAIL (hard_float, TARGET_HARD_FLOAT)
 
 /* RISCV_FTYPE_ATYPESN takes N RISCV_FTYPES-like type codes and lists
    their associated RISCV_ATYPEs.  */
+#define RISCV_FTYPE_ATYPES0(A) \
+  RISCV_ATYPE_##A
 #define RISCV_FTYPE_ATYPES1(A, B) \
   RISCV_ATYPE_##A, RISCV_ATYPE_##B
 
 static const struct riscv_builtin_description riscv_builtins[] = {
-  DIRECT_BUILTIN (frflags, RISCV_USI_FTYPE_VOID, hard_float),
+  DIRECT_BUILTIN (frflags, RISCV_USI_FTYPE, hard_float),
   DIRECT_NO_TARGET_BUILTIN (fsflags, RISCV_VOID_FTYPE_USI, hard_float)
 };
 

gcc/config/riscv/riscv-ftypes.def

@@ -26,5 +26,5 @@ along with GCC; see the file COPYING3.  If not see
       LIST contains the return-type code followed by the codes for each
         argument type.  */
 
-DEF_RISCV_FTYPE (1, (USI, VOID))
+DEF_RISCV_FTYPE (0, (USI))
 DEF_RISCV_FTYPE (1, (VOID, USI))

gcc/testsuite/g++.target/riscv/frflags.C

@@ -0,0 +1,7 @@
+/* { dg-options "-O2 -march=rv32if -mabi=ilp32f" } */
+/* { dg-do compile } */
+
+int f()
+{
+  return __builtin_riscv_frflags();
+}