Select a strategy to mitigate speculative bounds check bypass (aka Spectre-PHT or Spectre V1).
This is an experimental option - execution of untrusted code is not supported by GraalVM CE.
The accepted values are:
                  None - No mitigations are used in JIT compiled code.
            AllTargets - Speculative execution on all conditional branch targets is
                         stopped using speculative execution barrier instructions.
          GuardTargets - Branch targets relevant to Java memory safety are instrumented
                         with barrier instructions. This option has less performance impact
                         than AllTargets. 
  NonDeoptGuardTargets - Same as GuardTargets, except that branches which deoptimize are not
                         protected since they can not be executed repeatedly and are thus less
                         likely to be successfully exploited in an attack.

                         
Note that all modes except "None" will also instrument branch target blocks containing UNSAFE memory accesses
with barrier instructions.