Skip empty sections when calculating a R2R PE file layout

[davidnguyen-tech]

This PR updates PEObjectWriter.

The changes are independent from #122511

Problem

R2R PE files were bloated with empty space between sections.

Cause

Although we didn't directly emit empty sections into the R2R PE file, we still allocated a 64 kB padding for each empty section on non-Windows platforms:

• In the LayoutSections method, the virtualAddress counter was incremented to include a 64 kB padding for all sections, including the empty ones
• This caused sizeOfImage to include unnecessary 64 kB padding for each empty section on non-Windows platforms.

Solution

• Modified LayoutSections to skip VA allocation in PE file layout for empty sections - after this change, we don't add 64 kB padding before the empty sections anymore.

Results

Observed assembly: System.Web.dll

Original assembly size: 16 kB

After compiling to Composite R2R on Linux & Mac:

dotnet/runtime version	System.Web.dll R2R Image Size
release/10.0	4 kB
main	918 kB
main with this PR applied	600 kB (~35% decrease)
#122511 branch	262 kB
#122511 branch with this PR applied	70 kB (~73% decrease)

[Copilot]

Pull request overview

This PR optimizes R2R PE file layout by skipping virtual address allocation for empty sections, reducing unnecessary file size bloat.

Key changes:

• Modified LayoutSections to skip VA allocation and size calculations for empty sections
• Empty sections now receive zero VA/size in the output layout instead of consuming 64 kB padding
• Maintains section index correspondence by still adding entries to _outputSectionLayout for empty sections