[AArch 64] Use a constant pool load for weak symbol references when
using static relocation model and small code model.

Summary: currently we generate GOT based relocations for weak symbol
references regardless of the underlying relocation model. This should
be change so that in static relocation model we use a constant pool
load instead.

Patch from: Keith Walker

Reviewers: Renato Golin, Tim Northover

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217503 91177308-0d34-0410-b5e6-96231b3b80d8
[AArch64] Add experimental PBQP support

This adds target specific support for using the PBQP register allocator on the
AArch64, for the A57 cpu.

By default, the PBQP allocator is not used, unless explicitely required
on the command line with "-aarch64-pbqp".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217504 91177308-0d34-0410-b5e6-96231b3b80d8