AArch64: don't drop GlobalAddress offset when handling extern_weak decls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176258 91177308-0d34-0410-b5e6-96231b3b80d8
AArch64: be more careful resorting to inefficient addressing for weak vars.

If an otherwise weak var is actually defined in this unit, it can't be
undefined at runtime so we can use normal global variable sequences (ADRP/ADD)
to access it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176259 91177308-0d34-0410-b5e6-96231b3b80d8