arm64: Rename ARM64_SSBD to ARM64_SPECTRE_V4

In a similar manner to the renaming of ARM64_HARDEN_BRANCH_PREDICTOR
to ARM64_SPECTRE_V2, rename ARM64_SSBD to ARM64_SPECTRE_V4. This isn't
_entirely_ accurate, as we also need to take into account the interaction
with SSBS, but that will be taken care of in subsequent patches.

Signed-off-by: Will Deacon <will@kernel.org>