Change KLEIDICV_ENABLE_SVE2 behaviour

Previously KLEIDICV_ENABLE_SVE2 and KLEIDICV_ENABLE_SVE2_SELECTIVELY
were mutually exclusive. Now KLEIDICV_ENABLE_SVE2 enables or disables
SVE2 code paths overall, and KLEIDICV_LIMIT_SVE2_TO_SELECTED_ALGORITHMS
is a refinement of that option.

The intent of this change is merely to make the options more ergonomic.
It's now only necessary to set
KLEIDICV_LIMIT_SVE2_TO_SELECTED_ALGORITHMS=OFF in order to enable all
SVE2 code paths, instead of setting both
KLEIDICV_ENABLE_SVE2_SELECTIVELY=OFF and KLEIDICV_ENABLE_SVE2=ON.