[PATCH] Move -fbuiltin from c.opt to common.opt and change it to common group

[Kito Cheng]

Hi Richard:

>> -fno-builtin is seem not only for the c family front-end, but also
>> used in LTO now, so move it to common.opt and change it to `Common`.
>
> Please leave it in c-family and just add LTO to the set of supported
> languages.  -fno-builtin isn't meaningful for other frontends
> and we just happen to use the flag.
> If then it makes more sense to move -fhosted and -ffreestanding
> though I don't know how meaningful those are for other frontends.
>
> Or create a "proper" flag to communicate that the middle-end
> should avoid creating new calls to builtins at all cost
> (well, that's really what -ffreestanding is about).

-fno-builtin is meaningless for other front-end but middle-end,
However `-fno-builtin` is more explicit than -fhosted and  -ffreestanding,
when people see the option `-fno-builtin`, they will know what this
option mean "do not use builtin implicitly", and most important
 -fno-builtin is more well known than -fhosted or -ffreestanding.

and the flag_no_builtin is already used in gcc/lto/lto-lang.c:def_builtin_1,
so my patch is not first user of this option in LTO front-end.

> Richard.