libffi: refer to documentation of correct GCC version

[Xi Ruoyao]

Now we have GCC-8.2.0 but a link in libffi page still refers to GCC-8.1.0 doc. We should do

sed '72 s@8\.1\.0@&gcc-version;@' -i chapter06/libffi.xml

So it will always refer to the documentation of the GCC version corresponding to the LFS book.

[Bruce Dubbs]

Fixed at revision 11477.

I really don't need a sed for simple fixes to xml files. This was easy via vim. For extensive changes, a patch would be OK.