SPMI: TFM should not be hardcoded for microbenchmark collections

[jakobbotsch]

We hardcode the TFM for micro benchmarks in https://github.com/dotnet/runtime/blob/main/src/coreclr/scripts/superpmi_benchmarks.py, which requires us to manually update the value. @am11 suggests a way to avoid this in #109992 (comment).

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