Minor change

[Matthew Fluet]

> Another minor fix: with the native mode version of MLton, `-keep a' doesn't
> leave any .s or .S files around, and it clearly should do that for the natively
> generated assembler code.  I would argue that it should not leave around the
> .s version of the small C code it makes, but it should leave the .c file around,
> which it doesn't.

Well, under the last release of MLton -keep a didn't keep the .c or .s
files from the C backend.  My understanding was that the sets of files
kept by the different flags were all disjoint; -keep a would keep just
those auxilary files make up through the cps simplification phase, and a
few other representation decisions made in backend.fun.  I agree that it
would be useful to have a save "what MLton passes on to gcc" flag (which
would be the small .c and the .n.S files) but that would not save the
files generated by gcc (the small .s files and the .o files).