Dropping support for Kokkos_{DEVICES,OPTIONS,ARCH} in CMake

[dalg24]

We will only support -DKokkos_ENABLE_FOO=ON. It means no more two ways to do the same thing. We decided we would rather pay the price of verbosity rather than complexity of the build system.

[jeffmiles63]

We need to make sure that we keep consistent with the Kokkos Requirements. The Kokkos requirement is there be at least one host execution space. OpenMP with Serial is a legitimate configuration ... However, so is just OpenMP ... Serial should ONLY be enabled if it is explicitly asked for [-DKokkos_ENABLE_SERIAL=ON], or if there is not a host execution space present.

Here is an example of how this would look in kokkos_enable_devices.cmake ...
Turn on serial if we cannot find an active host execution space

IF (NOT KOKKOS_ENABLE_OPENMP)
IF (NOT KOKKOS_ENABLE_PTHREAD)
IF (NOT KOKKOS_ENABLE_HPX)
MESSAGE(STATUS "Setting Kokkos_ENABLE_SERIAL=ON because we need a host execution environment")
KOKKOS_DEVICE_OPTION(SERIAL ON "Enable serial backend because no other host backend present")
ENDIF()
ENDIF()
ENDIF()

[jjwilke]

@jeffmiles63 Will make sure this behavior is correct.

[jjwilke]

I will leave all arch variables as Kokkos_ARCH_. We will debate naming on a subsequent PR/issue.