Selects a strategy to mitigate speculative bounds check bypass (also known as Spectre-PHT or Spectre V1). 
The accepted values are:
                  None - No mitigations are used in JIT compiled code.
            AllTargets - Speculative execution on all branch targets is
                         stopped using speculative execution barrier instructions.
                         This option is equivalent to setting SpeculativeExecutionBarriers to true.
          GuardTargets - Branch targets relevant to Java memory safety are instrumented with barrier instructions. 
                         This option has a lower performance impact than AllTargets. 
  NonDeoptGuardTargets - Same as GuardTargets, except that branches which deoptimize are not protected because they cannot be 
                         executed repeatedly and are thus less likely to be successfully exploited in an attack.

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