GH-126491: GC: Mark objects reachable from roots before doing cycle collection

[markshannon]

This is an updated version of #126502

It differs as follows:

• The removal of lazy dict untracking has already been merged, so isn't present
• More care is taken to ensure the consistency of the spaces
• Some adjustments to the "work to do" calculation

• Issue: Mark all objects reachable from roots as live before doing main cyclic GC pass #126491

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit 698abb3 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[markshannon]

!buildbot Android

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @markshannon for commit 6d8a0d4 🤖

The command will test the builders whose names match following regular expression: Android

The builders matched are:

• AMD64 Android PR
• aarch64 Android PR