Support new upstream SGX driver in Linux kernels >5.11

[jovanbulck]

SGX driver has finally been upstreamed into recent Linux kernels, meaning the dependence on linux-sgx-driver (/dev/isgx) could eventually go, see:

https://lwn.net/Articles/840129/
https://www.kernel.org/doc/html/latest/x86/sgx.html

Should check how dev/sgx-step relies on the legacy /dev/isgx driver and ensure compatibility with the new upstreamed /dev/sgx_enclave driver (probably want to support both drivers for now).

[jovanbulck]

Some notes for future reference:

The driver /dev/sgx-step currently only relies on the /dev/isgx legacy driver in the following ways:

• discover enclave base address by retrieving the sgx_encl object from the vma->vm_private_data field -- while this appears to still be the same in the upstream /dev/sgx_enclave driver, we can probably do this cleaner from user space by simply querying the base address of the shared library enclave ELF file(?)
• support EDBGRD/WR by calling vma->vm_ops->access -- while we can always call the ENCLS leaves ourselves, doing it via the vma operations fields may still be preferable to not have to worry about page faults etc; the upstream driver's sgx_vma_access appears to do this the same way, so this "should" work out of the box hopefully w the new driver as well

edit for future reference: both requirements can probably be fulfilled without relying on driver specifics using standard Linux interfaces under /proc/self/maps and /proc/self/mem