[BOLT] Add code padding verification

Summary:
In non-relocation mode, we allow data objects to be embedded in the
code. Such objects could be unmarked, and could occupy an area between
functions, the area which is considered to be code padding.

When we disassemble code, we detect references into the padding area
and adjust it, so that it is not overwritten during the code emission.
We assume the reference to be pointing to the beginning of the object.

However, assembly-written functions may reference the middle of an
object and use negative offsets to reference data fields. Thus,
conservatively, we reduce the possibly-overwritten padding area to
a minimum if the object reference was detected.

Since we also allow functions with unknown code in non-relocation mode,
it is possible that we miss references to some objects in code.
To cover such cases, we need to verify the padding area before we
allow to overwrite it.

(cherry picked from FBD16477787)