Unix: don't spawn 'uname -m' when we can get the ProcessorArchitecture from the runtime. (#5897)
Unix: don't spawn 'uname -m' when we can get the ProcessorArchitecture from the runtime. (#5897)

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

This change will eliminate those.

sean-gilliam/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,mono/msbuild,mono/msbuild,mono/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild