Bgc suspension fixes (dotnet/coreclr#27729)

Bgc suspension fixes (dotnet/coreclr#27729)

* Changes to set gen0 bricks always. This reduces the time spent in find_first_object when finding the start of objects for marking interior pointers.

* Revert "Changes to set gen0 bricks always. This reduces the time spent in find_first_object when finding the start of objects for marking interior pointers."

This reverts commit dotnet/coreclr@9d53ff910ceb5482c67350a88e6687b8f94d5634.

* Two fixes to speed up suspension for foreground GCs while background GC is in progress:

- In background_mark_simple1, check g_fSuspensionPending and if it is set, save the state of the work and restart the loop - this will call allow_fgc() and thus allow a foreground GC to take place.

- In revisit_written_page, call allow_fgc() at the end - this allow a foreground GC to happen whenever we are done with revisiting a written page.

* Addressed code review feedback - use counter instead of testing g_fSuspensionPending directly.

Commit migrated from https://github.com/dotnet/coreclr/commit/a7678f5d8e14ad0edc8d996ae4048079c7b2d575