Remove some warnings and cleaning code (#1736)

Ryujinx.Common/SystemInfo/LinuxSystemInfo.cs

@@ -1,8 +1,10 @@
 using System.IO;
 using System.Linq;
+using System.Runtime.Versioning;
 
 namespace Ryujinx.Common.SystemInfo
 {
+    [SupportedOSPlatform("linux")]
     internal class LinuxSysteminfo : SystemInfo
     {
         public override string CpuName { get; }

Ryujinx.Common/SystemInfo/MacOSSysteminfo.cs

@@ -1,11 +1,13 @@
 using System;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 using System.Text;
 using Ryujinx.Common.Logging;
 
 namespace Ryujinx.Common.SystemInfo
 {
+    [SupportedOSPlatform("macos")]
     internal class MacOSSysteminfo : SystemInfo
     {
         public override string CpuName { get; }

Ryujinx.Common/SystemInfo/WindowsSystemInfo.cs

@@ -2,9 +2,11 @@ using Ryujinx.Common.Logging;
 using System;
 using System.Management;
 using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
 
 namespace Ryujinx.Common.SystemInfo
 {
+    [SupportedOSPlatform("windows")]
     internal class WindowsSysteminfo : SystemInfo
     {
         public override string CpuName { get; }