bfd/doc: use abs_srcdir when creating symlinks

bfd/doc: use abs_srcdir when creating symlinks

After commit:

  commit bd32be01c997f686ab0b53f0640eaa0aeb61fbd3
  Date:   Fri Dec 3 00:23:20 2021 -0500

      bfd: merge doc subdir up a level

And the follow-up commit:

  commit 98b1464bdf6306a8ab4614b5e9f76cdb2dd00b33
  Date:   Wed Oct 2 22:58:08 2024 +0300

      bfd: fix unnecessary bfd.info regen

There is still a problem building the bfd docs from a release tar
file.

As the release tar file contains the pre-generated .texi files we
expect the bfd/doc build stage to symlink to the pre-existing .texi
files in the source tree.

However, this is still not working as expected if $(srcdir) is
relative.  The problem is this line in REGEN_TEXI:

    test -e $$texi || test ! -f $(srcdir)/$$texi || $(LN_S) $(srcdir)/$$texi $$texi; \

This is executed from the build/bfd/ directory, so if $(srcdir) is
relative, then this will get you from the bfd/ directory in the build
tree to the corresponding bfd/ directory in the src tree.  However,
the symlink is created in the bfd/doc/ build directory.  The relative
path will then fail to take you to the bfd/ directory in the src
tree.

Fix this by using $(abs_srcdir) when creating the symlink.

Approved-By: Nick Clifton <nickc@redhat.com>