Add riscv64gc-unknown-freebsd

Author: Tobias Kortkamp

std/src/os/raw/mod.rs

@@ -69,7 +69,8 @@ type_alias! { "char.md", c_char = u8, NonZero_c_char = NonZeroU8;
             target_arch = "aarch64",
             target_arch = "arm",
             target_arch = "powerpc",
-            target_arch = "powerpc64"
+            target_arch = "powerpc64",
+            target_arch = "riscv64"
         )
     ),
     all(
@@ -112,7 +113,8 @@ type_alias! { "char.md", c_char = i8, NonZero_c_char = NonZeroI8;
             target_arch = "aarch64",
             target_arch = "arm",
             target_arch = "powerpc",
-            target_arch = "powerpc64"
+            target_arch = "powerpc64",
+            target_arch = "riscv64"
         )
     ),
     all(