[X86] Use proxy scheduler models for bdver3/bdver4 cpus (#114873)

We don't have specific models for bdver3/bdver4 cpus but we can use the
bdver2/znver1 models as proxy standins - these days the models are more
useful for analysis than for perfect instruction scheduling so these
should be fine.

While they don't accurately represent the bdver3/bdver4 architecture
(specifically the different fp-pipe layout), they give more accurate
latency/throughputs (vs Agner) than the default SandyBridge model, and
enable PostRA scheduling which all recent AMD models have benefitted
from.

I had to use the znver1 model for bdver4 so that we have AVX2
instruction coverage (none of the TBM/XOP/LWP/FMA4 instructions have
explicit schedules so this shouldn't be a problem) - they both
double-pump 256-bit instructions so this works pretty well.

This patch is based off a discussion at the devmtg regarding how easily
we can provide an actual scheduler model (or at least approximation) to
more of the X86 cpu targets - we can then add specific models if the
(unlikely) need arises.

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