The code that copies code bytes from a MacroAssembler into a buffer in a
CodeRangeUnwindingRecord struct (used to store stack unwinding data) has an
error: it copies the whole MacroAssembler buffer size, not just the size of the
compiled instructions into an "exception thunk" array.
This has no real bad effects, because a CodeRangeUnwindingRecord is stored at
the beginning of a page reserved at the beginning of an isolate code range, but
it is quite bad and we need to fix it.

Bug: v8:3598
Change-Id: I0df0cf0173561cc939e6431bc0f01ef040fc189e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1629310Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Paolo Severini <paolosev@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#61863}