[ELF/AArch64] Add support for external TLS variables

[zatrazz]

Summary

This patch add support to shared library TLS variable in AArch64RelocationPass
handleGOTTPREL creation and also reword the method comment.

[ruiu]

LGTM

lib/ReaderWriter/ELF/AArch64/AArch64RelocationPass.cpp

257–258

Maybe these two lines can fit in 80 columns?

lib/ReaderWriter/ELF/AArch64/AArch64RelocationPass.cpp

 #endif
       _gotMap[atom] = g;
       _gotVector.push_back(g);
       return g;
     }
     return got->second;
   }
 
-  /// \brief Create a GOT TPREL entry and change the relocation to a PC32 to
-  /// the GOT.
+  /// \brief Create a GOT TPREL entry to local or external TLS variable.
   std::error_code handleGOTTPREL(const Reference &ref) {
-    if (isa<DefinedAtom>(ref.target()))
+    if (isa<DefinedAtom>(ref.target()) ||
+        isa<SharedLibraryAtom>(ref.target()))
       const_cast<Reference &>(ref).setTarget(getGOTTPREL(ref.target()));
     return std::error_code();
   }
 
   /// \brief Generates a double GOT entry with R_AARCH64_TLSDESC dynamic
   /// relocation reference.  Since the dynamic relocation is resolved
   /// lazily so the GOT associated should be in .got.plt.
   const GOTAtom *getTLSDESCPLTEntry(const Atom *da) {