Add check to make sure mold supports riscv

contributing/development/compiling/compiling_for_linuxbsd.rst

@@ -383,12 +383,15 @@ To cross-compile Godot for RISC-V devices, we need to setup the following items:
   make sure you see ``riscv64`` on the list of targets.
 - `mold <https://github.com/rui314/mold/releases>`__. This fast linker,
   is the only one that correctly links the resulting binary. Download it, extract it,
-  and make sure to add its ``bin`` folder to your PATH.
+  and make sure to add its ``bin`` folder to your PATH. Run
+  ``mold --help | grep support`` to check if your version of Mold supports RISC-V.
+  If you don't see RISC-V, your Mold may need to be updated.
 
 To make referencing our toolchain easier, we can set an environment
 variable like this:
 
 ::
+
     export RISCV_TOOLCHAIN_PATH="path to toolchain here"
 
 This way, we won't have to manually set the directory location