This is part of implementing a technique to mitigate against Spectre v1,
similar in spirit to what has been proposed by Chandler for X86_64 at
This patch enables not tracking miss-speculation status, but instead
using a more heavy weight DSB + ISB barrier to implement the semantics
of the llvm.speculation_safe_value intrinsic. This may be beneficial
e.g. when only an infrequently executed part of the code needs to be
protected using manually inserted intrinsics.