There are a few parts to this change:

1. Ensure that MSB3270 and MSB3277 warnings are promoted to errors in CI
and hence block merging.
1. Move our DiaSymReader.Native logic into a separate targets file.
This resource cannot be consumed with a simple package reference but
rather requires a package reference and custom logic to pull out the
contained binaries. This logic used to be spread through our build. Now
it's in a single place.
1. Remove the x86 bootstrapping logic. This was testing a pretty obscure
scenario and the cost of maintaining that logic is siginificant at this
point. Can bring back if we ever find a bug in this area.

The root cause of the MSB3270 warnings is a subtle change in the SDK. It
now passes the runtime graph to NuGet for native assets. In the case of
DiaSymReader.Native there are runtime specific assets hence the SDK /
NuGet had to pick one for framework projects. This eventually lead to
`PlatformTarget` being set to x86 where it shoud have been AnyCPU.

Part of the change includes adding `ExcludeAssets=all` to the package
reference which means they no longer figure into this logic and hence
the binaries are marked as AnyCPU.

This regression in behavior is being tracked by
https://github.com/dotnet/sdk/issues/3495