Spectre will be dealt with via a Safari update (11.0.2 is the patched version). As far as I know that should work fine on Sierra.
As far as I can tell, there has not yet been a patch for Sierra or El Capitan for Meltdown (I'd expect one at some point, since neither are "legacy" yet), but High Sierra is already patched - insofar as anything is patched for these issues.
This vulnerabilities are tricky to deal with since they are not so much bugs as flaws in the design philosophy or all (well, most) modern chips.
Not sure how much that helps, but maintaining good security practices remains the best advice.