compile crossgen2 itself with crossgen2 and trimming.

[mangod9]

Now that crossgen2 has replaced original crossgen would be good to enable precompilation + trimming for the tool to reduce the package size.

[AntonLapounov]

We do precompile crossgen2; however, it is not trim-compatible at present, because it uses COM to invoke the native DiaSymReader in order to generate PDBs.

[AntonLapounov]

WIP: #56787

[davidwrighton]

This is now done. We actually compile with NativeAOT which is even better.