Access to the constant pool of off-heap builtins must use
Instruction{Start,Size} instead of the raw instruction_{start,size}
accessors, and we need to copy the constant_pool_offset field when
creating trampolines.

This in turn required access to the embedded blob without an
associated isolate, which is now implemented by global variable set by
each isolate. Both writes and reads are relaxed, as races do not
matter since each isolate will attempt to set the same value of the
blob and its size.

Drive-by: Support off-heap code disassembly.

Bug: v8:6666,v8:7575
Change-Id: I4f203acd4dc128339cf2dd54b3253d9552616649
Reviewed-on: https://chromium-review.googlesource.com/973442
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52209}