compiler: increase buffer size to avoid -Wformat-length warning

    
    GCC will soon get a new -Wformat-length option as described in
    https://gcc.gnu.org/PR49905.  This change makes a buffer larger to avoid
    a warning.
    
    Reviewed-on: https://go-review.googlesource.com/28915

Also revert ChangeLog entry in gcc/go/ChangeLog--changes to
gcc/go/gofrontend do not get ChangeLog entries.

From-SVN: r240054