Spectre mitigation

[Demi Obenour]

The recent “Spectre” bug requires that speculative execution of indirect
branches be disabled.  For GHC, this will require passing a flag to LLVM
and fixing the NCG to emit suitable calling sequences.

This will be a disaster for the STG execution model, because it disables
CPU branch prediction for indirect calls and jumps.  This is a big argument
in favor of doing a CPS→SSA conversion in the backend.
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