[lld-macho] Support EH frames under arm64

For arm64, llvm-mc emits relocations for the target function
address like so:

  ltmp:
    <CIE start>
    ...
    <CIE end>
    ... multiple FDEs ...
    <FDE start>
    <target function address - (ltmp + pcrel offset)>
    ...

If any of the FDEs in `multiple FDEs` get dead-stripped, then `FDE start`
will move to an earlier address, and `ltmp + pcrel offset` will no longer
reflect an accurate pcrel value. To avoid this problem, we "canonicalize"
our relocation by adding an `EH_Frame` symbol at `FDE start`, and updating
the reloc to be `target function address - (EH_Frame + new pcrel offset)`.

Reviewed By: #lld-macho, Roger

Differential Revision: https://reviews.llvm.org/D124561