[LLD][AArch64] Increase alignment of AArch64AbsLon… (#247)

This permits an AArch64AbsLongThunk to be used in an environment where
unaligned accesses are disabled.

The AArch64AbsLongThunk does a load of an 8-byte address. When unaligned
accesses are disabled this address must be 8-byte aligned.

The vast majority of AArch64 systems will have unaligned accesses
enabled in userspace. However, after a reset, before the MMU has been
enabled, all memory accesses are to "device" memory, which requires
aligned accesses. In systems with multi-stage boot loaders a thunk may
be required to a later stage before the MMU has been enabled.

As we only want to increase the alignment when the ldr is used we delay
the increase in thunk alignment until we know we are going to write an
ldr. We also need to account for the ThunkSection alignment increase
when this happens.

In some of the test updates, particularly those with shared CHECK lines
with position independent thunks it was easier to ensure that the thunks
started at an 8-byte aligned address in all cases.

This patch applies llvm-project PR 133738 to the arm-software release/20.x branch.

Author: smithp35