When #92921 added the `__arm_get_current_vg` functionality, it used the
FMV feature bits mechanism rather than the mechanism that was previously
added for SME which called `getauxval` on Linux platforms or
`__aarch64_sme_accessible` required for baremetal libraries. It is
better to always use `__aarch64_cpu_features`.
For baremetal we still need to rely on `__arm_sme_accessible` to
initialise the struct.
cb4f4a8a4d