SLS Mitigation patches backported for GCC9

[Matthew Malcomson]

Hello,

Eventually we will want to backport the SLS patches to older branches.

When the GCC10 release is unfrozen we will work on getting the same patches
already posted backported to that branch.  The patches already posted on the
mailing list apply cleanly to the current releases/gcc-10 branch.

I've heard interest in having the GCC 9 patches, so I'm posting the modified
versions upstream sooner than otherwise.

Cheers,
Matthew

Entire patch series attached to cover letter.
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