[BOLT-AArch64] Fix -icf, -use-old-text and -update-debug-sections

Summary:
Refactor MCInst comparison code to support target-dependent
functionality. This was necessary because AArch64 uses MCTargetExprs
that only the AArch64 backend knows how to unpack it and compare. Also
fix a bug where a relocation against a constant island would make BOLT
create a fixed reference against a code location in a similar way to
read-only data, so when we asked to -use-old-text, the code would break
for this particular HHVM function
(_ZN5folly2io4zlib18defaultZlibOptionsEv) because the reference now
contains invalid data, since the original .text was overwritten. Finally,
fix a bug with -update-debug-sections on AArch64 where the update
loop wasn't expecting a function with zero basic blocks, which can
happen on AArch64 because some functions contain just a constant
island.

(cherry picked from FBD7679244)