Enable Elf::at<T> only for POD data types

Summary:
- Make sure that the utility function Elf::at<T> only works for POD
datatypes, as it uses reinterpret_cast<T>.

Test Plan: - Unit tests

Reviewed By: simpkins@fb.com

FB internal diff: D748314

folly/experimental/symbolizer/Elf.h

@@ -123,7 +123,8 @@ class ElfFile {
   void validateStringTable(const ElfW(Shdr)& stringTable) const;
 
   template <class T>
-  const T& at(off_t offset) const {
+  const typename std::enable_if<std::is_pod<T>::value, T>::type&
+  at(off_t offset) const {
     return *reinterpret_cast<T*>(file_ + offset);
   }