Add an option so perf jit dump can optionally use TSC for timestamps.

[Kuinox]

I'm trying to make perf with Intel PT works with RyuJIT.
It currently doesn't work because Intel PT use TSC as a timestamp, and RyuJIT uses the monotonic clock for the dump timestamp.

The change would read the environment variable flag JITDUMP_USE_ARCH_TIMESTAMP, and when this flag is true, change the behavior of GetTimeStampNS to use __rdtsc, and also set the file header flag JITDUMP_FLAGS_ARCH_TIMESTAMP

I'm open to do the contribution, but I have next to no experience with C++.

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[jkotas]

This issue about Linux perf tool support. Historically, this fell into area-Diagnostics (for example, check dotnet/coreclr#26897, #229, #99986, ...). @tommcdon Is it intentional that we want Linux perf tool support to be tracked under CodeGen now?

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[tommcdon]

This issue about Linux perf tool support. Historically, this fell into area-Diagnostics (for example, check dotnet/coreclr#26897, #229, #99986, ...). @tommcdon Is it intentional that we want Linux perf tool support to be tracked under CodeGen now?

Thanks Jan! Moving to dotnet-diagnostics-coreclr - I had originally read this issue as being an RyuJIT request