Add RISCV64 TargetArchitecture

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It is for supporting RISCV64 in cecil
Add RISCV64 to TargetArchitecture and update checks for pe64

Part of dotnet/runtime#84834
cc @dotnet/samsung

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@jbevain Could you please review this PR? It is for RISC-V architecture. Thank you.

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I collected error message in CI.
Is it caused by this PR? If then, could you please give any guide for me to fix the error?
Thank you.

Testhost process for source(s) '/home/runner/work/cecil/cecil/symbols/pdb/Test/bin/Debug/netcoreapp3.1/Mono.Cecil.Pdb.Tests.dll' exited with error: You must install or update .NET to run this application.
Testhost process for source(s) '/home/runner/work/cecil/cecil/rocks/Test/bin/Debug/netcoreapp3.1/Mono.Cecil.Rocks.Tests.dll' exited with error: You must install or update .NET to run this application.
Testhost process for source(s) '/home/runner/work/cecil/cecil/symbols/mdb/Test/bin/Debug/netcoreapp3.1/Mono.Cecil.Mdb.Tests.dll' exited with error: You must install or update .NET to run this application.
Testhost process for source(s) '/home/runner/work/cecil/cecil/symbols/pdb/Test/bin/Debug/netcoreapp3.1/Mono.Cecil.Pdb.Tests.dll' exited with error: You must install or update .NET to run this application.
Error: Process completed with exit code 1.

@jbevain Could you please help? Thank you.

[Numpsy]

Is it caused by this PR? If then, could you please give any guide for me to fix the error?

Looks like the unit tests are trying to run on .NET Core 3.1, and the build agents don't have that installed by default any more?

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My issue fixed by dotnet/runtime#101038