docs: fix invalid footnote syntax

All footnotes must come after a separator in reStructuredText.  Fix the
two files in which this does not happen.

This mistake causes the link to be rendered literally:

   ...from the venv itself[#distlib]_. If no...

and is caught by Sphinx 8.1.0 as an unreferenced footnote.

Reviewed-by: Peter Maydell <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

docs/devel/atomics.rst

@@ -204,7 +204,7 @@ They come in six kinds:
   before the second with respect to the other components of the system.
   Therefore, unlike ``smp_rmb()`` or ``qatomic_load_acquire()``,
   ``smp_read_barrier_depends()`` can be just a compiler barrier on
-  weakly-ordered architectures such as Arm or PPC[#]_.
+  weakly-ordered architectures such as Arm or PPC\ [#]_.
 
   Note that the first load really has to have a _data_ dependency and not
   a control dependency.  If the address for the second load is dependent

docs/devel/build-system.rst

@@ -145,13 +145,13 @@ was installed in the ``site-packages`` directory of another interpreter,
 or with the wrong ``pip`` program.
 
 If a package is available for the chosen interpreter, ``configure``
-prepares a small script that invokes it from the venv itself[#distlib]_.
+prepares a small script that invokes it from the venv itself\ [#distlib]_.
 If not, ``configure`` can also optionally install dependencies in the
 virtual environment with ``pip``, either from wheels in ``python/wheels``
 or by downloading the package with PyPI.  Downloading can be disabled with
 ``--disable-download``; and anyway, it only happens when a ``configure``
 option (currently, only ``--enable-docs``) is explicitly enabled but
-the dependencies are not present[#pip]_.
+the dependencies are not present\ [#pip]_.
 
 .. [#distlib] The scripts are created based on the package's metadata,
               specifically the ``console_script`` entry points.  This is the