RISC-V: Change constexpr back to CONSTEXPR

According to f95d3d5de7.
Since GCC 4.8.6 doesn't support constexpr, we should change it back to CONSTEXPR.
gcc/ChangeLog:

	* config/riscv/riscv-vector-builtins-bases.cc: Change constexpr back to CONSTEXPR.
	* config/riscv/riscv-vector-builtins-shapes.cc (SHAPE): Ditto.
	* config/riscv/riscv-vector-builtins.cc (struct registered_function_hasher): Ditto.
	* config/riscv/riscv-vector-builtins.h (struct rvv_arg_type_info): Ditto.

gcc/config/riscv/riscv-vector-builtins-bases.cc

@@ -84,8 +84,8 @@ public:
   }
 };
 
-static constexpr const vsetvl<false> vsetvl_obj;
-static constexpr const vsetvl<true> vsetvlmax_obj;
+static CONSTEXPR const vsetvl<false> vsetvl_obj;
+static CONSTEXPR const vsetvl<true> vsetvlmax_obj;
 namespace bases {
 const function_base *const vsetvl = &vsetvl_obj;
 const function_base *const vsetvlmax = &vsetvlmax_obj;

gcc/config/riscv/riscv-vector-builtins-shapes.cc

@@ -71,7 +71,7 @@ build_all (function_builder &b, const function_group_info &group)
 /* Declare the function shape NAME, pointing it to an instance
    of class <NAME>_def.  */
 #define SHAPE(DEF, VAR) \
-  static constexpr const DEF##_def VAR##_obj; \
+  static CONSTEXPR const DEF##_def VAR##_obj; \
   namespace shapes { const function_shape *const VAR = &VAR##_obj; }
 
 /* Base class for for build.  */

gcc/config/riscv/riscv-vector-builtins.cc

@@ -88,7 +88,7 @@ struct registered_function_hasher : nofree_ptr_hash<registered_function>
 };
 
 /* Static information about each RVV type.  */
-static constexpr const vector_type_info vector_types[] = {
+static CONSTEXPR const vector_type_info vector_types[] = {
 #define DEF_RVV_TYPE(NAME, NCHARS, ABI_NAME, ARGS...)                          \
   {#NAME, #ABI_NAME, "u" #NCHARS #ABI_NAME},
 #include "riscv-vector-builtins.def"
@@ -123,23 +123,23 @@ static const rvv_type_info i_ops[] = {
 #include "riscv-vector-builtins-types.def"
   {NUM_VECTOR_TYPES, 0}};
 
-static constexpr const rvv_arg_type_info rvv_arg_type_info_end
+static CONSTEXPR const rvv_arg_type_info rvv_arg_type_info_end
   = rvv_arg_type_info (NUM_BASE_TYPES);
 
 /* A list of args for size_t func (void) function.  */
-static constexpr const rvv_arg_type_info void_args[]
+static CONSTEXPR const rvv_arg_type_info void_args[]
   = {rvv_arg_type_info (RVV_BASE_void), rvv_arg_type_info_end};
 
 /* A list of args for size_t func (size_t) function.  */
-static constexpr const rvv_arg_type_info size_args[]
+static CONSTEXPR const rvv_arg_type_info size_args[]
   = {rvv_arg_type_info (RVV_BASE_size), rvv_arg_type_info_end};
 
 /* A list of none preds that will be registered for intrinsic functions.  */
-static constexpr const predication_type_index none_preds[]
+static CONSTEXPR const predication_type_index none_preds[]
   = {PRED_TYPE_none, NUM_PRED_TYPES};
 
 /* A static operand information for size_t func (void) function registration. */
-static constexpr const rvv_op_info i_none_size_void_ops
+static CONSTEXPR const rvv_op_info i_none_size_void_ops
   = {i_ops,				/* Types */
      OP_TYPE_none,			/* Suffix */
      rvv_arg_type_info (RVV_BASE_size), /* Return type */
@@ -147,7 +147,7 @@ static constexpr const rvv_op_info i_none_size_void_ops
 
 /* A static operand information for size_t func (size_t) function registration.
  */
-static constexpr const rvv_op_info i_none_size_size_ops
+static CONSTEXPR const rvv_op_info i_none_size_size_ops
   = {i_ops,				/* Types */
      OP_TYPE_none,			/* Suffix */
      rvv_arg_type_info (RVV_BASE_size), /* Return type */

gcc/config/riscv/riscv-vector-builtins.h

@@ -171,7 +171,7 @@ struct rvv_builtin_suffixes
 /* RVV Builtin argument information.  */
 struct rvv_arg_type_info
 {
-  constexpr rvv_arg_type_info (rvv_base_type base_type_in)
+  CONSTEXPR rvv_arg_type_info (rvv_base_type base_type_in)
     : base_type (base_type_in)
   {}
   enum rvv_base_type base_type;