[lldb] Skip TestTargetXMLArch if no support for x86 target

If LLVM is configured without X86 as one of its TARGETS_TO_BUILD, then lldb
will crash when using X86 disassembler (which it does while running `image
show-unwind`).

lldb/test/API/functionalities/gdb_remote_client/TestTargetXMLArch.py

@@ -130,6 +130,7 @@ class TestTargetXMLArch(GDBRemoteTestBase):
 
     @skipIfXmlSupportMissing
     @skipIfRemote
+    @skipIfLLVMTargetMissing("X86")
     def test_register_augmentation(self):
         """
         Test that we correctly associate the register info with the eh_frame