Enable DOTNET_ThreadGuardPages opt-in config setting on 32-bit

[tommcdon]

Moves DOTNET_ThreadGuardPages config check out of the 64-bit host check, making the number of pages we save on stackoverflow on 32-bit configurable.

[Copilot]

Pull Request Overview

This PR moves the DOTNET_ThreadGuardPages configuration check out of the 64-bit-only block so that 32-bit hosts can opt into custom guard pages instead of always using the default.

• Inverts the if so non-zero config applies on all platforms
• Retains default guard pages only for 64-bit when config is zero
• Keeps a single debug page on 32-bit regardless of config

Comments suppressed due to low confidence (3)

src/coreclr/vm/threads.cpp:6143

• Add tests to verify the new opt-in behavior on 32-bit: ensure zero (default) yields no extra guard pages and positive values apply correctly.

        int ThreadGuardPages = CLRConfig::GetConfigValue(CLRConfig::EXTERNAL_ThreadGuardPages);

src/coreclr/vm/threads.cpp:6141

• [nitpick] Update the public configuration docs to note that DOTNET_ThreadGuardPages now applies as an opt-in setting on 32-bit hosts as well.

#endif // HOST_64BIT

src/coreclr/vm/threads.cpp:6144

• Consider using ThreadGuardPages > 0 instead of != 0 to prevent negative values from reducing the guard size unexpectedly.

        if (ThreadGuardPages != 0)

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[janvorli]

LGTM, thank you!