[LLD][ELF][AArch64] Add AArch64 short range thunk support

The AArch64 branch immediate instruction has a 128MiB range. This
makes it suitable for use a short range thunk in the same way as
short thunks are implemented in Arm and PPC. This patch adds
support for short range thunks to AArch64.

Adding short range thunk support should mean that OutputSections
can grow to nearly 256 MiB in size without needing long-range
indirect branches.

Differential Revision: https://reviews.llvm.org/D148701