This is a reland of c5379162

The reland fixes Code::clear_padding to correctly clear trailing
padding.

Original change's description:
> [code] Move the unwinding info into metadata area
>
> 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/+/2484795
> Reviewed-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}

Cq-Include-Trybots: luci.v8.try:v8_linux64_msan_rel_ng
Tbr: leszeks@chromium.org
Bug: v8:11036
Change-Id: I2ea056fe2a53217e0b5ae25661b92f5ddec6fca5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2485501
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70645}