bfd: fix unnecessary bfd.info regen

bfd: fix unnecessary bfd.info regen

When building from an unmodified release tarball, a REGEN_TEXI
invocation is supposed to create a symlink to the .texi file
in the source directory and discard the newly generated .tmp file.
However, after commit bd32be01c997 ("bfd: merge doc subdir up a level")
it creates the symlink at the wrong level, and then a .texi with
a fresh timestamp, which in turn forces bfd.info regeneration.
This breaks builds in environments without makeinfo program.

Fix this by creating the symlink at the level of the target stamp.

Fixes: bd32be01c997 ("bfd: merge doc subdir up a level")
Signed-off-by: Alexey Izbyshev <izbyshev@ispras.ru>