Prune some unnecessary code from Monitor.Enter/Exit in single-threaded wasm builds of corlib

Author: kg

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -24,6 +24,8 @@
     <IsBigEndian Condition="'$(Platform)' == 's390x'">true</IsBigEndian>
     <Is64Bit Condition="'$(Platform)' == 'arm64' or '$(Platform)' == 'x64' or '$(Platform)' == 's390x' or '$(Platform)' == 'loongarch64' or '$(Platform)' == 'ppc64le' or '$(Platform)' == 'riscv64'">true</Is64Bit>
     <UseMinimalGlobalizationData Condition="'$(TargetsBrowser)' == 'true' or '$(TargetsWasi)' == 'true'">true</UseMinimalGlobalizationData>
+    <FeatureWasmManagedThreads Condition="'$(WasmEnableThreads)' == 'true'">true</FeatureWasmManagedThreads>
+    <DefineConstants Condition="'$(FeatureWasmManagedThreads)' == 'true'">$(DefineConstants);FEATURE_WASM_MANAGED_THREADS</DefineConstants>
   </PropertyGroup>
   <PropertyGroup>
     <DefineConstants Condition="'$(IsBigEndian)' == 'true'">$(DefineConstants);BIGENDIAN</DefineConstants>
@@ -2766,4 +2768,4 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
-</Project>
+</Project>

src/mono/System.Private.CoreLib/src/System/Threading/Monitor.Mono.cs

@@ -30,10 +30,13 @@ public static void Exit(object obj)
         {
             if (obj == null)
                 ArgumentNullException.ThrowIfNull(obj);
+#if TARGET_BROWSER && !FEATURE_WASM_MANAGED_THREADS
+#else
             if (ObjectHeader.TryExitChecked(obj))
                 return;
 
             InternalExit(obj);
+#endif
         }
 
         public static bool TryEnter(object obj)
@@ -157,13 +160,16 @@ private static void ReliableEnterTimeout(object obj, int timeout, ref bool lockT
             if (timeout < 0 && timeout != (int)Timeout.Infinite)
                 throw new ArgumentOutOfRangeException(nameof(timeout));
 
+#if TARGET_BROWSER && !FEATURE_WASM_MANAGED_THREADS
+#else
             // fast path
             if (ObjectHeader.TryEnterFast(obj)) {
                 lockTaken = true;
                 return;
             }
 
             try_enter_with_atomic_var(obj, timeout, true, ref lockTaken);
+#endif
         }
 
         public static long LockContentionCount => Monitor_get_lock_contention_count() + Lock.ContentionCount;