Handle _GLOBAL_OFFSET_TABLE_ in 64 bit mode.

With this MC is able to handle _GLOBAL_OFFSET_TABLE_ in 64 bit mode, which is
needed for medium and large code models.

This fixes pr19470.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206793 91177308-0d34-0410-b5e6-96231b3b80d8