gnu: openmpi: Configure without builtin atomics.

See https://www.mail-archive.com/users@lists.open-mpi.org//msg31400.html



* gnu/packages/mpi.scm (openmpi)[arguments]: Don't configure with
--enable-builtin-atomics.

Signed-off-by: Ludovic Courtès <ludovic.courtes@inria.fr>