AArch64: Allow offsets to be folded into addresses with ELF.

This is a code size win in code that takes offseted addresses
frequently, such as C++ constructors that typically need to compute
an offseted address of a vtable. It reduces the size of Chromium for
Android's .text section by 46KB, or 56KB with ThinLTO (which exposes
more opportunities to use a direct access rather than a GOT access).

Because the addend range is limited in COFF and Mach-O, this is
enabled for ELF only.

Differential Revision: https://reviews.llvm.org/D45199

llvm-svn: 329611