[ELF][AArch64] Add support for AArch64 range thunks.

The AArch64 unconditional branch and branch and link instructions have a
maximum range of 128 Mib. This is usually enough for most programs but
there are cases when it isn't enough. This change adds support for range
extension thunks to AArch64. For pc-relative thunks we follow the small
code model and use ADRP, ADD, BR. This has a limit of 4 gigabytes.

Differential Revision: https://reviews.llvm.org/D39744

llvm-svn: 319307