Add support for NetBSD/riscv64 aka. riscv64gc-unknown-netbsd.

Author: he32

core/src/ffi/mod.rs

@@ -132,7 +132,12 @@ mod c_char_definition {
             ),
             all(
                 target_os = "netbsd",
-                any(target_arch = "aarch64", target_arch = "arm", target_arch = "powerpc")
+                any(
+                    target_arch = "aarch64",
+                    target_arch = "arm",
+                    target_arch = "powerpc",
+                    target_arch = "riscv64"
+                )
             ),
             all(
                 target_os = "vxworks",