Added an internal "hpc" target to the makefile which builds with MPI and all currently available backends.
Added an internal "hpc" target to the makefile which builds with MPI and
all currently available backends.

We may choose to advertise this later, but for now it is mostly for
developers and testing.

mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem,mfem/mfem