Exit with non-zero status when Aborting!: out of memory...

Exit with non-zero status when Aborting!: out of memory...

...when run with --batch-mode.  This correctly signals to make(1) that
there was an error.

I used nearest-cmdl/batch-mode? though I wonder what a "batch level"
is.  Such a thing should probably cmdl-interrupt/abort-previous rather
than %exit, but this short-circuits the whole gc-clean recovery
effort.  I find similar yuck (a call to %exit) in ttyio.scm's
operation/read-char.