Optional jump table support

[Alan-Jowett]

eBPF for Windows requires support for updating the address of helper functions (when the module hosting them is reloaded).

This is currently implemented as a jump table in eBPF-for-Windows, but the performance is sub-part. A better solution would be to modify uBPF to emit rip indirect calling like this, with the jump table being embedded in the emitted machine code.

call   QWORD PTR [rip+0x2023]        # 0x2029

This might also eliminate the need to perform retpoline based calling.

[Alan-Jowett]

Resolved by: #427