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
977d62c33e