onnxruntime

Rework some external targets to ease building with `-DFETCHCONTENT_FULLY_DISCONNECTED=ON` (#15323)

### Description
Rework some external targets to ease building with
`-DFETCHCONTENT_FULLY_DISCONNECTED=ON`
This will allow package managers to more easily provide an onnxruntime
package by reducing the amount of patching needed downstream at each
version.

### Motivation and Context
Availability of onnxruntime in some C++ package managers
https://github.com/microsoft/onnxruntime/issues/7150
https://github.com/conan-io/conan-center-index/issues/16699
https://github.com/microsoft/vcpkg/issues/20548

My initial intent is to get this in conan but the PR would most likely
be useful (though not tested) to vcpkg as well (and maybe others).
I tried to get only a first batch of not too specific patches (i.e. not
specific to conan).

The first commit reworks `flatbuffers` and just extends what @snnn did
in https://github.com/microsoft/onnxruntime/pull/13991
The second commit reworks `pytorch_cpuinfo`
The third commit reworks `google_nsync`