bfd: use $(LN_S) in favor of "cp -p" when populating pre-built *.texi

bfd: use $(LN_S) in favor of "cp -p" when populating pre-built *.texi

"cp -p" has been observed to fail on Cygwin when the build tree is on a
local drive but the sources are on a Samba share. We don't really need
full copies of the files here - symlinks suffice.