Update openfoam (#11866)

* OpenFOAM: rationalize architecture options

- older versions of OpenFOAM had WM_ARCH_OPTION to choose a 32-bit
  build on 64-bit architectures. This is not exactly relevant anymore
  and has now been largely removed from OpenFOAM.

- simultaneously the newest version of OpenFOAM has rationalized the
  handling of special settings for KNL by rolling these into the
  compiler options. We now do the same thing here in spack.

  1. Reuse the internal variable arch_option to convey processor-specific
     optimizations and add these into the C++OPT=... statement.
  2. Drop spack support for configuring a 32-bit build on 64-bit
     hardware.

These changes can also be applied to older OpenFOAM versions, and to
various OpenFOAM forks without issue.

* update openfoam versions. New release 1906, patched 1812 version.