fix

Author: pavelsavara

src/libraries/Common/src/System/Net/StreamBuffer.cs

@@ -132,8 +132,6 @@ public int WriteBytesAvailable
 
         public void Write(ReadOnlySpan<byte> buffer)
         {
-            if (!Thread.IsThreadStartSupported) throw new PlatformNotSupportedException();
-
             if (buffer.Length == 0)
             {
                 return;

src/libraries/System.Private.CoreLib/src/System/Threading/Lock.cs

@@ -309,8 +309,6 @@ private void ExitImpl()
         [MethodImpl(MethodImplOptions.NoInlining)]
         private ThreadId TryEnterSlow(int timeoutMs, ThreadId currentThreadId)
         {
-            if (!Thread.IsThreadStartSupported) throw new PlatformNotSupportedException();
-
             Debug.Assert(timeoutMs >= -1);
 
             if (!currentThreadId.IsInitialized)
@@ -394,6 +392,8 @@ private ThreadId TryEnterSlow(int timeoutMs, ThreadId currentThreadId)
                 spinCount = maxSpinCount;
             }
 
+            if (!Thread.IsThreadStartSupported) throw new PlatformNotSupportedException(); // this should not happen, but it helps to trim the code below on ST platforms
+
             for (short spinIndex = 0; ;)
             {
                 LowLevelSpinWaiter.Wait(spinIndex, SpinSleep0Threshold, isSingleProcessor: false);
@@ -483,6 +483,8 @@ private ThreadId TryEnterSlow(int timeoutMs, ThreadId currentThreadId)
                 goto Locked;
             }
 
+            if (!Thread.IsThreadStartSupported) throw new PlatformNotSupportedException(); // this should not happen, but it helps to trim the code below on ST platforms
+
             // Lock was not acquired and a waiter was registered. All following paths need to unregister the waiter, including
             // exceptional paths.
             try