Minor change

[Henry Cejtin]

I was thinking that something like `g' as a new keep letter which would keep
the `generated' code.  I.e., the C code generated by MLton and the assembler
code generated directly by MLton (not the result of gcc running on C code
generated by MLton) would be a reasonable choice.  I could definitely live with
the -keep cS way of doing it though.
Any reaction to the reduced -v?  I.e., I would like something that shows me
the various other programs being run (gcc and as) but not all the other stuff.
Also I don't think that the current -v shows the calls to assemble generated
code in the native case (generated .s/.S stuff).