[SOL] Allow selectively disable compiler builtins for BPF target (#4)

* [BPF] Make rust allocation builtins unavailable for BPF target

- the calls to rust alloc/dealloc routines need to be preserved and
not replaced by compiler generated builtin inlined code

Author: dmakarov

llvm/lib/Analysis/TargetLibraryInfo.cpp

@@ -864,6 +864,12 @@ static void initialize(TargetLibraryInfoImpl &TLI, const Triple &T,
     TLI.setUnavailable(LibFunc_vec_free);
   }
 
+  if (T.isBPF()) {
+    TLI.setUnavailable(LibFunc_rust_alloc);
+    TLI.setUnavailable(LibFunc_rust_dealloc);
+    TLI.setUnavailable(LibFunc_rust_realloc);
+  }
+
   TLI.addVectorizableFunctionsFromVecLib(ClVectorLibrary, T);
 }