[BOLT-AArch64] Support relocation mode for bzip2

Summary:
As we deal with incomplete addresses in address-computing
sequences of code in AArch64, we found it is easier to handle them in
relocation mode in the presence of relocations.

Incomplete addresses may mislead BOLT into thinking there are
instructions referring to a basic block when, in fact, this may be the
base address of a data reference. If the relocation is present, we can
easily spot such cases.

This diff contains extensions in relocation mode to understand and
deal with AArch64 relocations. It also adds code to process data inside
functions as marked by AArch64 ABI (symbol table entries named "$d").
In our code, this is called constant islands handling. Last, it extends
bughunter with a "cross" mode, in which the host generates the binaries
and the user test them (uploading to the target), useful when debugging
in AArch64.

(cherry picked from FBD6024570)