Disable large memory Span::Clear test on Linux (for 2.0) (#20736)

* Disable large memory Span::Clear test on Linux

* Patching the comment for this location

Author: shiftylogic

src/System.Memory/tests/Span/Clear.cs

@@ -226,8 +226,13 @@ public static void ClearValueTypeWithReferences()
             Assert.Equal<TestValueTypeWithReference>(expected, actual);
         }
 
+        // NOTE: ClearLongerThanUintMaxValueBytes test is constrained to run on Windows and MacOSX because it causes
+        //       problems on Linux due to the way deferred memory allocation works. On Linux, the allocation can
+        //       succeed even if there is not enough memory but then the test may get killed by the OOM killer at the
+        //       time the memory is accessed which triggers the full memory allocation.
         [Fact]
         [OuterLoop]
+        [PlatformSpecific(TestPlatforms.Windows | TestPlatforms.OSX)]
         unsafe static void ClearLongerThanUintMaxValueBytes()
         {
             if (sizeof(IntPtr) == sizeof(long))