Allow front end to override maintenance of errno

[craig@jcb-sc.com]

>Ok to commit, per earlier discussions?  It doesn't affect code generation
>of any front end except (currently) g77.

One side effect of this patch I just noticed, is that the
g77.f-torture/compile/19990502-0.f test case no longer fails.

Since that failure almost certainly indicated a bug in the back end,
I heartily recommend somebody knowledgable debug it anyway, if necessary
(after I committ this patch, assuming it's approved) by undoing the
pertinent egcs/gcc/f/com.c patchlet.

        tq vm, (burley)