Add no-op futex syscall implementation

[abejgonzalez]

This allows simple programs compiled with modern versions of GLIBC to work with pk. For example:

hello.cc:

#include <iostream>
int main(void) {
  std::cout << "Hello!" << std::endl;
  return 0;
}

Compile/test:

riscv64-unknown-linux-gnu-gcc hello.cc -lstdc++ -static -o hello
spike pk hello

Solution was previously given in:
#207
riscv-software-src/riscv-isa-sim#1443
riscv-software-src/riscv-isa-sim#1605

[abejgonzalez]

Re-using sys_stub_success instead of creating a new function.

[jerryz123]

I think this is fine.... it doesn't behave exactly the same as sys_futex, but that should be ok