We need to check to see if the application wanting to be executed is in
the rootfs first. Otherwise we end up in a case where we either lose
tracking of applications in FEX, or applications fail to launch since
they don't exist in the global filesystem
Sadly these things can't be split without breaking functionality so it
turns in to a bit of a mess.
SyscallHandler is very much something that is a Linux only construct and
shouldn't be in FEXCore itself. Lets the frontend register a
Syscallhandler with FEXCore. FEXCore itself is then aware of the current
syscall ABI and handles the ABI in an optimal fashion.
So it is not a 100% clean break otherwise we would lose performance.
The SignalDelegator then needs to move to the frontend since the
SyscallHandler requires it for signal based syscalls.
The CPU backend signal handling still needs to happen in FEXCore because
it is a very tight coupling with the CPU backend.
Once we need to support more Signal handling we can give the backends
cleaner support to select which specific OS handler to handle.
