[browser][coreCLR] enable [UnmanagedCallersOnly]

[pavelsavara]

• for CorHost2::CreateDelegate in c++
• for SystemJS_InstallTimerCallback((delegate* unmanaged[Cdecl]<void>)&TimerHandler); in c#

Related https://github.com/dotnet/runtime/pull/120583/files#r2422365823
Related #120976

cc @AaronRobinsonMSFT

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