Fix and subsequently rewrite Spectre mitigations, including the addition
of support for PR_SPEC_DISABLE_NOEXEC.

(Will Deacon and Marc Zyngier)
* for-next/ghostbusters: (22 commits)
  arm64: Add support for PR_SPEC_DISABLE_NOEXEC prctl() option
  arm64: Pull in task_stack_page() to Spectre-v4 mitigation code
  KVM: arm64: Allow patching EL2 vectors even with KASLR is not enabled
  arm64: Get rid of arm64_ssbd_state
  KVM: arm64: Convert ARCH_WORKAROUND_2 to arm64_get_spectre_v4_state()
  KVM: arm64: Get rid of kvm_arm_have_ssbd()
  KVM: arm64: Simplify handling of ARCH_WORKAROUND_2
  arm64: Rewrite Spectre-v4 mitigation code
  arm64: Move SSBD prctl() handler alongside other spectre mitigation code
  arm64: Rename ARM64_SSBD to ARM64_SPECTRE_V4
  arm64: Treat SSBS as a non-strict system feature
  arm64: Group start_thread() functions together
  KVM: arm64: Set CSV2 for guests on hardware unaffected by Spectre-v2
  arm64: Rewrite Spectre-v2 mitigation code
  arm64: Introduce separate file for spectre mitigations and reporting
  arm64: Rename ARM64_HARDEN_BRANCH_PREDICTOR to ARM64_SPECTRE_V2
  KVM: arm64: Simplify install_bp_hardening_cb()
  KVM: arm64: Replace CONFIG_KVM_INDIRECT_VECTORS with CONFIG_RANDOMIZE_BASE
  arm64: Remove Spectre-related CONFIG_* options
  arm64: Run ARCH_WORKAROUND_2 enabling code on all CPUs
  ...