[LLDB] Xfail TestVSCode_eventStatistic.py on Arm/AArch64 Linux

This patch marks TestVSCode_eventStatistic.py as xfail on Arm/AArch64
Linux platform. test_initialized_event testcase is failing. See buildbot
logs below:

https://lab.llvm.org/buildbot/#/builders/17/builds/30199
https://lab.llvm.org/buildbot/#/builders/96/builds/31528

Differential Revision: https://reviews.llvm.org/D138077