arm64: Support systems without FP/ASIMD

The arm64 kernel assumes that FP/ASIMD units are always present
and accesses the FP/ASIMD specific registers unconditionally. This
could cause problems when they are absent. This patch adds the
support for kernel handling systems without FP/ASIMD by skipping the
register access within the kernel. For kvm, we trap the accesses
to FP/ASIMD and inject an undefined instruction exception to the VM.

The callers of the exported kernel_neon_begin_partial() should
make sure that the FP/ASIMD is supported.

Cc: Will Deacon <[email protected]>
Cc: Christoffer Dall <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Signed-off-by: Suzuki K Poulose <[email protected]>
Reviewed-by: Marc Zyngier <[email protected]>
[[email protected]: add comment on the ARM64_HAS_NO_FPSIMD conflict and the new location]
Signed-off-by: Catalin Marinas <[email protected]>

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