[BOLT][AArch64] Run LDR relaxation (#165787)
Replace the current `ADRRelaxationPass` with `AArch64RelaxationPass`,
which, besides the existing ADR relaxation, will also run LDR relaxation
that for now only handles these two forms of LDR instructions:
`ldr Xt, [label]` and `ldr Wt, [label]`.
