BPF in GCC

FYI: people might be wondering what "functions that need to do their own save and restore of the register values" might be. The answer is varargs functions. GCC's testsuite calls a *lot* of these: it has many thousands of calls to printf alone. Obviously you won't want to use these in the kernel, but since the tests are not testing printf itself, it would be nice to be able to run them regardless.