contrib/bc: upgrade to version 6.7.6
This update fixes a potential issue when flushing stdout on exit
fails: longjmp could use an uninitialized target address variable.
Most files are included in this commit due to a changed date in
the copyright note.
(cherry picked from commit 52a5ec1b178fd07651446c7e31b1512794a04dbf)
MFC after: 3 days