KVM: arm64: Reorganise CPTR trap manipulation

The NVHE/HVHE and VHE modes have separate implementations of
__activate_cptr_traps() and __deactivate_cptr_traps() in their
respective switch.c files. There's some duplication of logic, and it's
not currently possible to reuse this logic elsewhere.

Move the logic into the common switch.h header so that it can be reused,
and de-duplicate the common logic.

This rework changes the way SVE traps are deactivated in VHE mode,
aligning it with NVHE/HVHE modes:

* Before this patch, VHE's __deactivate_cptr_traps() would
  unconditionally enable SVE for host EL2 (but not EL0), regardless of
  whether the ARM64_SVE cpucap was set.

* After this patch, VHE's __deactivate_cptr_traps() will take the
  ARM64_SVE cpucap into account. When ARM64_SVE is not set, SVE will be
  trapped from EL2 and below.

The old and new behaviour are both benign:

* When ARM64_SVE is not set, the host will not touch SVE state, and will
  not reconfigure SVE traps. Host EL0 access to SVE will be trapped as
  expected.

* When ARM64_SVE is set, the host will configure EL0 SVE traps before
  returning to EL0 as part of reloading the EL0 FPSIMD/SVE/SME state.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250617133718.4014181-4-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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