spidermonkey: Force lp64d ABI for riscv64

cc-rs is hardcoded to use the soft-float ABI for riscv64, while
the rest of the system is double-float. This is not just a missing
stub header file - You can't link object files using different
ABIs.

pkgs/development/interpreters/spidermonkey/78.nix

@@ -107,6 +107,10 @@ stdenv.mkDerivation rec {
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];
 
+  # cc-rs insists on using -mabi=lp64 (soft-float) for riscv64,
+  # while we have a double-float toolchain
+  NIX_CFLAGS_COMPILE = lib.optionalString (with stdenv.hostPlatform; isRiscV && is64bit) "-mabi=lp64d";
+
   # Remove unnecessary static lib
   preFixup = ''
     moveToOutput bin/js78-config "$dev"