AK: The <cxxabi.h> header is not available during Toolchain build

This will need some refinement, but basically since we build LibC
during the toolchain build, we don't have libstdc++ headers yet.

AK/Demangle.h

@@ -27,18 +27,25 @@
 #pragma once
 
 #include <AK/String.h>
+
+#ifndef SERENITY_LIBC_BUILD
 #include <cxxabi.h>
+#endif
 
 namespace AK {
 
 inline String demangle(const StringView& name)
 {
+#ifdef SERENITY_LIBC_BUILD
+ return name;
+#else
  int status = 0;
  auto* demangled_name = abi::__cxa_demangle(String(name).characters(), nullptr, nullptr, &status);
  auto string = String(status == 0 ? demangled_name : name);
  if (status == 0)
  kfree(demangled_name);
  return string;
+#endif
 }
 
 }