Unix: don't spawn 'uname -m' when we can get the ProcessorArchitecture from the runtime.

[tmds]

No description provided.

[tmds]

Most commands I run with dotnet cli that involve msbuild spawn one or more uname -m child processes.

For example:

$ strace -qq -f -e execve dotnet build
execve("/usr/bin/dotnet", ["dotnet", "build"], 0x7ffc4faa3748 /* 59 vars */) = 0
[pid 22919] execve("/usr/lib64/dotnet/dotnet", ["dotnet", "exec", "/usr/lib64/dotnet/sdk/5.0.100-pr"..., "-maxcpucount", "-verbosity:m", "-restore", "-consoleloggerparameters:Summary"], 0x561504fa6a90 /* 64 vars */) = 0
Microsoft (R) Build Engine version 16.8.0-preview-20414-02+a55ce4fbb for .NET
Copyright (C) Microsoft Corporation. All rights reserved.

[pid 22934] execve("/usr/bin/uname", ["uname", "-m"], 0x565242790720 /* 65 vars */) = 0
  Determining projects to restore...
[pid 22947] execve("/usr/bin/uname", ["uname", "-m"], 0x7f15102a0640 /* 65 vars */) = 0
  All projects are up-to-date for restore.
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  console -> /tmp/console/bin/Debug/net5.0/console.dll

This change will eliminate those.

cc @rainersigwald

[tmds]

I've added aarch64 case here.

[rainersigwald]

Looks great, thanks!

[rainersigwald]

PR build hang was #5520, I'll rerun.

[rainersigwald]

@tmds our builds are currently broken due to an Arcade infrastructure issue and we have been asked to prioritize getting #5879 into 16.9 preview 2, so we'll merge this as soon as everything's working again and that's started.