cl/396621747: ghost: add SGS to the global_scalability benchmark
cl/396621747: ghost: add SGS to the global_scalability benchmark

Since SGS spits out some text, you can now redirect the test output to a file:

  ./global_scalability -o /tmp/foo sgs

Note that global_scalability still needs some work.  It's based on certain
assumptions about how global agents work: namely that they can only schedule a
cpu once per loop.  With BPF, that is no longer true.  Fun tidbit: SGS,
untuned, does better than SOL at low numbers of cpus, and the machine is mostly
idle.

Tested: global_scalability on SKX

google/ghost-userspace,google/ghost-userspace,google/ghost-userspace,google/ghost-userspace