add RiscV64 support, fixes dotnet#2644 (dotnet#2647)

src/BenchmarkDotNet/Environments/Platform.cs

@@ -51,5 +51,10 @@ public enum Platform
         /// A PowerPC 64-bit (little-endian) processor architecture.
         /// </summary>
         Ppc64le,
+
+        /// <summary>
+        /// A RiscV 64-bit processor architecture.
+        /// </summary>
+        RiscV64,
     }
 }

src/BenchmarkDotNet/Portability/RuntimeInformation.cs

@@ -207,12 +207,13 @@ internal static Runtime GetCurrentRuntime()
         public static Platform GetCurrentPlatform()
         {
             // these are not part of .NET Standard 2.0, so we use hardcoded values taken from
-            // https://github.com/dotnet/runtime/blob/1a37caf773a3b857ccff49a31be3333d4fdc491f/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Architecture.cs#L9
+            // https://github.com/dotnet/runtime/blob/080fcae7eaa8367abf7900e08ff2e52e3efea5bf/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/Architecture.cs#L9
             const Architecture Wasm = (Architecture)4;
             const Architecture S390x = (Architecture)5;
             const Architecture LoongArch64 = (Architecture)6;
             const Architecture Armv6 = (Architecture)7;
             const Architecture Ppc64le = (Architecture)8;
+            const Architecture RiscV64 = (Architecture)9;
 
             switch (ProcessArchitecture)
             {
@@ -234,6 +235,8 @@ public static Platform GetCurrentPlatform()
                     return Platform.Armv6;
                 case Ppc64le:
                     return Platform.Ppc64le;
+                case RiscV64:
+                    return Platform.RiscV64;
                 default:
                     throw new ArgumentOutOfRangeException();
             }