Shadow stack is part of Intel's Control-Flow Enforcement Technology (CET).
Whenever a function is called, the return address is pushed onto both
the regular stack and the shadow stack. When that function returns, the
return addresses are popped off both stacks and compared; if they fail
to match, #CP raised.
With this commit, we create shadow stack for each fiber context and
switch the shadow stack accordingly during fcontext switch.
Signed-off-by: Chen, Hu <hu1.chen@intel.com>
Closes GH-9283.
