GCStress is not safe when not using per-thread allocation contexts

[swgillespie]

When investigating https://github.com/dotnet/coreclr/issues/10265 I noticed that GCHeap::StressHeap does not acquire the alloc lock when allocating strings. When not using per-thread allocation contexts (e.g. Windows x86/x64 single proc scenarios), this is not safe. Another thread can be allocating through the normal allocation path while another thread is allocating through GCHeap::StressHeap, resulting in random heap corruption.

The correct fix is to acquire the global allloc lock before calling GCHeap::StressHeap when running without per-thread allocation contexts. I don't believe anyone is currently running GC stress on single-proc machines, so I imagine this is pretty low priority to fix.

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