lldb: Run TestCrossDSOTailCalls.py and TestCrossObjectTailCalls.py on Darwin only

See https://bugs.llvm.org/show_bug.cgi?id=44561, these tests are failing
on an aarch64/Linux bot:

http://lab.llvm.org:8011/builders/lldb-aarch64-ubuntu/builds/655

For some reason the backtrace the tests are expecting to find is
incomplete.

Author: vedantk

lldb/packages/Python/lldbsuite/test/functionalities/tail_call_frames/cross_dso/TestCrossDSOTailCalls.py

@@ -17,7 +17,7 @@ def setUp(self):
 
     @skipIf(compiler="clang", compiler_version=['<', '8.0'])
     @skipIf(dwarf_version=['<', '4'])
-    @expectedFailureAll(oslist=["windows"], bugnumber="llvm.org/pr26265")
+    @skipUnlessDarwin # llvm.org/PR44561
     def test_cross_dso_tail_calls(self):
         self.build()
         exe = self.getBuildArtifact("a.out")

lldb/packages/Python/lldbsuite/test/functionalities/tail_call_frames/cross_object/TestCrossObjectTailCalls.py

@@ -17,7 +17,7 @@ def setUp(self):
 
     @skipIf(compiler="clang", compiler_version=['<', '8.0'])
     @skipIf(dwarf_version=['<', '4'])
-    @expectedFailureAll(oslist=["windows"], bugnumber="llvm.org/pr26265")
+    @skipUnlessDarwin # llvm.org/PR44561
     def test_cross_object_tail_calls(self):
         self.build()
         exe = self.getBuildArtifact("a.out")