[Bug driver/101447] Remove legacy external declarations in toplev.h

[ashimida at linux dot alibaba.com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101447

--- Comment #3 from ashimida <ashimida at linux dot alibaba.com> ---
Submitted at https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575252.html