llvmPackages_{20,21,git}.compiler-rt: no fmv on aarch64 without libc

aarch64 FMV (Function MultiVersioning) is not available before the libc
is available. This causes issues while building with compiler-rt on
aarch64. By disabling FMV before the libc is available resolves missing
symbol errors.

pkgs/development/compilers/llvm/common/compiler-rt/default.nix

@@ -244,6 +244,10 @@ stdenv.mkDerivation (finalAttrs: {
   ++ lib.optionals (noSanitizers && lib.versionAtLeast release_version "19") [
     (lib.cmakeBool "COMPILER_RT_BUILD_CTX_PROFILE" false)
   ]
+  ++
+    lib.optional (stdenv.hostPlatform.isAarch64 && !haveLibc)
+      # Fixes https://github.com/NixOS/nixpkgs/issues/393603
+      (lib.cmakeBool "COMPILER_RT_DISABLE_AARCH64_FMV" true)
   ++ devExtraCmakeFlags;
 
   outputs = [