For technical reasons, we used to serialize off-heap trampolines with a
RelocInfo containing a single OFF_HEAP_TARGET entry. This RelocInfo was
never needed at runtime. It also ended up being somewhat misleading
because printing an embedded code object would show the OFF_HEAP_TARGET
entry (belonging to the trampoline) at the first 'real' instruction.

With this CL, we explicitly serialize an empty byte array as the reloc
info for each off-heap trampoline, i.e. the snapshot will never contain
such off-heap target reloc infos.

Bug: v8:6666, v8:7969
Change-Id: If6fa85a438d093ed5dcea07ce0de1db49a224d28
Reviewed-on: https://chromium-review.googlesource.com/1146643
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54786}