Add riscv{32,64} crossSystems.

lib/systems/examples.nix

@@ -68,6 +68,14 @@ rec {
   musl64 = { config = "x86_64-unknown-linux-musl"; };
   musl32  = { config = "i686-unknown-linux-musl"; };
 
+  riscv = bits: {
+    config = "riscv${bits}-unknown-linux-gnu";
+    platform = platforms.riscv-multiplatform bits;
+  };
+  riscv64 = riscv "64";
+  riscv32 = riscv "32";
+
+
   #
   # Darwin
   #

lib/systems/platforms.nix

@@ -541,6 +541,12 @@ rec {
     };
   };
 
+  riscv-multiplatform = bits: {
+    name = "riscv-multiplatform";
+    kernelArch = "riscv";
+    bfdEmulation = "elf${bits}lriscv";
+  };
+
   selectBySystem = system: {
       "i686-linux" = pc32;
       "x86_64-linux" = pc64;