[BUG] Segfault when SVC/SVR and LinearSVC coexist under cuml.accel

[betatim]

Using SVC (or SVR) in the same process as LinearSVC (or LinearSVR) under cuml.accel causes a segfault. This was discovered while adding ONNX export support (#7805).

The crash appears to be triggered by the interaction between the two estimator types at the C++/CUDA layer, not by ONNX itself. The ONNX export tests work individually for each estimator, but combining LinearSVC and SVC in the same script crashes.

To reproduce this remove the comments for SVR/SVC and run the script in https://gist.github.com/betatim/d28503b15177a4530534117187b568de

SVC and SVR ONNX export tests are marked xfail in test_onnx.py (the _gamma private attribute is also missing from the proxy, but the segfault is the more severe issue).

Found this while working on #7805

The goal of this issue is to remove the xfail for these estimators in test_onnx.py. If we fix it we also need to update docs/source/cuml-accel/limitations.rst.