[Bug bootstrap/88623] gcc build uses CXX_FOR_BUILD but files have .c extension

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88623

--- Comment #1 from joseph at codesourcery dot com <joseph at codesourcery dot 
com> ---
gcc/ is not libgcc.  libgcc is only ever built using exactly the same 
version of GCC.