Add build/config/riscv.gni

Required for building latest V8.

build/config/riscv.gni

@@ -0,0 +1,19 @@
+# Copyright 2023 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/v8_target_cpu.gni")
+
+if (current_cpu == "riscv64" || v8_current_cpu == "riscv64") {
+  declare_args() {
+    # RISCV Vector extension compilation flag.
+    riscv_use_rvv = false
+
+    # RISCV Vector extension VELEN. Possible values are:
+    #   128
+    #   256
+    #   512
+    #   1024
+    riscv_rvv_vlen = 128
+  }
+}