The handling of syscalls should be unified

[mquinson]

In https://github.com/mquinson/simterpose/blob/master/src/syscall_process.c#L1664 we have a very large switch case on the kind of syscall to be handled.

It would be much cleaner if the code handling each of them could be unified, with the {pre,post} functions merged together. I would prefer having one function per syscall in charge of handling both sides of the call.