Semantically, the unwinding info is a variable-size metadata table
with untagged (i.e. no relocation needed) contents, packed inside Code
objects. This is just like other metadata tables (safepoint table,
handler table, constant pool, code comments); but for historical
reasons it's been treated differently so far. Unlike these other
tables, the unwinding info was located *after* InstructionEnd, and its
size was written to the first 8 bytes after InstructionEnd.

This CL makes unwinding info handling more consistent with other
metadata tables by writing its offset into a dedicated
kUnwindingInfoOffsetOffset header slot, and by moving the actual data
inside the [InstructionStart,InstructionEnd[ area. In follow-up CLs,
this area will be split into dedicated instruction- and metadata
areas.

A picture is worth 1000 words, before:

 +--------------------------+  <-- raw_instruction_start()
 |       instructions       |
 |           ...            |
 +--------------------------+
 |     embedded metadata    |  <-- safepoint_table_offset()
 |           ...            |  <-- handler_table_offset()
 |                          |  <-- constant_pool_offset()
 |                          |  <-- code_comments_offset()
 |    padding to the next   |
 |  8-byte aligned address  |
 +--------------------------+  <-- raw_instruction_end()
 |   [unwinding_info_size]  |
 |        as uint64_t       |
 +--------------------------+  <-- unwinding_info_start()
 |       unwinding info     |
 |            ...           |
 +--------------------------+  <-- unwinding_info_end()

After:

 +--------------------------+  <-- raw_instruction_start()
 |       instructions       |
 |           ...            |
 +--------------------------+
 |     embedded metadata    |  <-- safepoint_table_offset()
 |           ...            |  <-- handler_table_offset()
 |                          |  <-- constant_pool_offset()
 |                          |  <-- code_comments_offset()
 |                          |  <-- unwinding_info_offset()
 |                          |
 +--------------------------+  <-- raw_instruction_end()

Bug: v8:11036
Change-Id: I649708821acc5365186ca2c9cff2669fc3e91fd3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2484795Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70640}