llvm{9,11}: Disable tests on RISC-V

The test suite leads to nothing but tears, sorrow, and wasted build
time. It probably should be disabled for all of them but doing only
9 (llvmPackages) and 11 (llvmPackages_latest, Rust) for now. Some of
the failures have been fixed in LLVM main:

- https://reviews.llvm.org/D97490
- https://reviews.llvm.org/D91043

pkgs/development/compilers/llvm/11/llvm/default.nix

@@ -177,7 +177,7 @@ in stdenv.mkDerivation (rec {
     cp NATIVE/bin/llvm-config $dev/bin/llvm-config-native
   '';
 
-  doCheck = stdenv.isLinux && (!stdenv.isx86_32) && (!stdenv.hostPlatform.isMusl);
+  doCheck = stdenv.isLinux && (!stdenv.isx86_32) && (!stdenv.hostPlatform.isMusl) && (!stdenv.hostPlatform.isRiscV);
 
   checkTarget = "check-all";
 

pkgs/development/compilers/llvm/9/llvm/default.nix

@@ -177,7 +177,7 @@ in stdenv.mkDerivation (rec {
     cp NATIVE/bin/llvm-config $dev/bin/llvm-config-native
   '';
 
-  doCheck = stdenv.isLinux && (!stdenv.isx86_32);
+  doCheck = stdenv.isLinux && (!stdenv.isx86_32) && (!stdenv.hostPlatform.isRiscV);
 
   checkTarget = "check-all";