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_ENABLE_SVE2_SELECTIVELY 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_ENABLE_SVE2_SELECTIVELY=OFF in order to enable all SVE2 code paths, instead of setting both KLEIDICV_ENABLE_SVE2_SELECTIVELY=OFF and KLEIDICV_ENABLE_SVE2=ON.