Fix g_gc_lowest_address updating

[janvorli]

There is a rare issue that occurs when virtual memory blocks allocated by GC end up getting addresses that trigger a code path where the g_gc_lowest_address .. g_gc_highest_address is expanded down and the g_gc_lowest_address is exactly 2/3 of the g_gc_highest_address. We end up setting the g_gc_lowest_address to 0 and that then breaks GC because it starts considering NULL references as being part of the GC heap.
The culprit is a check that uses > instead of >=, which prevents the g_gc_lowest_address to be clamped to OS_PAGE_SIZE as its minimal value.

Close #131216

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Pull request overview

Fixes an edge-case in CoreCLR GC card-table growth logic where expanding the tracked heap address range could incorrectly allow g_gc_lowest_address to become 0, which can cause GC to treat null references as in-heap.

Changes:

• Adjusts a boundary comparison from > to >= when deciding whether to clamp the new lowest address to OS_PAGE_SIZE, preventing the “exactly half” span case from underflowing to 0.

[kkokosa]

The PR body says the bug triggers when "g_gc_lowest_address  is exactly 1/2 of  g_gc_highest_address " (i.e.  L = H/2 ). But there's ps *= 2 at line 604. The real exact boundary is L = 2/3 * H. Just for the sake of documentation.

[janvorli]

/backport to release/10.0

[github-actions]

Started backporting to release/10.0 (link to workflow run)

[github-actions]

@janvorli backporting to release/10.0 failed, the patch most likely resulted in conflicts. Please backport manually!

git am output

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: Fix g_gc_lowest_address updating
Using index info to reconstruct a base tree...
A	src/coreclr/gc/card_table.cpp
Falling back to patching base and 3-way merge...
CONFLICT (modify/delete): src/coreclr/gc/card_table.cpp deleted in HEAD and modified in Fix g_gc_lowest_address updating.  Version Fix g_gc_lowest_address updating of src/coreclr/gc/card_table.cpp left in tree.
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 Fix g_gc_lowest_address updating
Error: The process '/usr/bin/git' failed with exit code 128

Link to workflow output