Recent changes to remove cpus_have_const_cap() introduced new users of
cpus_have_cap() in the period between detecting system cpucaps and
patching alternatives. It would be preferable to defer these until after
the relevant cpucaps have been patched so that these can use the usual
feature check helper functions, which is clearer and has less risk of
accidental usage of code relying upon an alternative which has not yet
been patched.

This patch reworks the system-wide cpucap detection and patching to
minimize this transient period:

* The detection, enablement, and patching of system cpucaps is moved
  into a new setup_system_capabilities() function so that these can be
  grouped together more clearly, with no other functions called in the
  period between detection and patching. This is called from
  setup_system_features() before the subsequent checks that depend on
  the cpucaps.

  The logging of TTBR0 PAN and cpucaps with a mask is also moved here to
  keep these as close as possible to update_cpu_capabilities().

  At the same time, comments are corrected and improved to make the
  intent clearer.

* As hyp_mode_check() only tests system register values (not hwcaps) and
  must be called prior to patching, the call to hyp_mode_check() is
  moved before the call to setup_system_features().

* In setup_system_features(), the use of system_uses_ttbr0_pan() is
  restored, now that this occurs after alternatives are patched. This is
  a partial revert of commit:

    53d62e99 ("arm64: Avoid cpus_have_const_cap() for ARM64_HAS_PAN")

* In sve_setup() and sme_setup(), the use of system_supports_sve() and
  system_supports_sme() respectively are restored, now that these occur
  after alternatives are patched. This is a partial revert of commit:

    a76521d1 ("arm64: Avoid cpus_have_const_cap() for ARM64_{SVE,SME,SME2,FA64}")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20231212170910.3745497-2-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>