[RESEND PATCH] Makeconfig: Set pie-ccflag to -fPIE by default

[Wilco Dijkstra]

Hi Fangrui,

> In Clang, -fpic/-fPIC without  -fno-semantic-interposition still allows
> interprocedural optimizations of default visibility external linkage definitions.
>
> It is unfortunate that GCC doesn't allow it and
> --enable-default-semantic-interposition is rejected (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100937)

I think it now does it with -Ofast, so that's a start at least...

> So if such an option -ffast-PIC is made, it will only benefit GCC.
> I have a long write-up in https://maskray.me/blog/2021-05-09-fno-semantic-interposition#in-action

Well if changing the default of fPIC is unacceptable in GCC, adding a new option
should help LLVM too since it could imply more than -fno-semantic-interposition.
A key question is whether we could require that exported symbols are marked
explicitly so that we only emit GOT and PLT indirections on such exported symbols.

Cheers,
Wilco