[TrimmableTypeMap] Add GetFunctionPointer(int) ordinal dispatch

[simonrozsival]

Part of #10799
Stacked on #10917.

Summary

Adds GetFunctionPointer(int methodIndex) to ACW proxy types. This method maps ordinal indices to [UnmanagedCallersOnly] wrapper function pointers, providing an alternative dispatch mechanism to RegisterNatives.

IL emission

Emits a chain of comparisons:

public IntPtr GetFunctionPointer (int methodIndex)
{
    if (methodIndex == 0) return &wrapper_0;
    if (methodIndex == 1) return &wrapper_1;
    // ...
    return IntPtr.Zero;
}

Each case uses ldftn to load the function pointer of the corresponding [UnmanagedCallersOnly] static method, using the same wrapperHandles dictionary populated during UCO emission.

Tests

• Verifies GetFunctionPointer method exists on generated ACW proxy types

[simonrozsival]

Closing — GetFunctionPointer is redundant since we use RegisterNatives to bind JNI native methods to UCO wrappers at class load time. The earlier spec mentioned GetFunctionPointer as an alternative to RegisterNatives (for LLVM IR trampolines), but we chose RegisterNatives instead.