Mark arm/aarch64 specific xfails with expectedFailureLinux decorator

This patch marks some known failures and puts on expectedFailureLinux decorator to have testsuite xfail them.

Affected tests are: 

test/functionalities/watchpoint/step_over_watchpoint.py
test/functionalities/watchpoint/watchpoint_set_command/TestWatchLocationWithWatchSet.py
test/tools/lldb-server/TestGdbRemoteSingleStep.py
test/tools/lldb-server/TestGdbRemote_vCont.py

llvm-svn: 258315