Embed libxml2 for linux, mac and windows

Issue: https://gitee.com/openharmony/manifest/issues/I7TS92?from=project-issue

Test: LLDB enables libxml2 testing
Signed-off-by: Nlijie0325 <2410599907@qq.com>

llvm-toolchain.xml

@@ -17,4 +17,5 @@
   <project name="third_party_libedit" path="third_party/libedit" />
   <project name="third_party_ncurses" path="third_party/ncurses" />
   <project name="third_party_python" path="third_party/python" />
+  <project name="third_party_libxml2" path="third_party/libxml2" />
 </manifest>