x86/mm: Ensure clear_page() variants always have __kcfi_typeid_ symbols

When building with CONFIG_CFI=y and CONFIG_LTO_CLANG_FULL=y, there is a series
of errors from the various versions of clear_page() not having __kcfi_typeid_
symbols.

  $ cat kernel/configs/repro.config
  CONFIG_CFI=y
  # CONFIG_LTO_NONE is not set
  CONFIG_LTO_CLANG_FULL=y

  $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 clean defconfig repro.config bzImage
  ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_rep
  >>> referenced by ld-temp.o
  >>>               vmlinux.o:(__cfi_clear_page_rep)

  ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_orig
  >>> referenced by ld-temp.o
  >>>               vmlinux.o:(__cfi_clear_page_orig)

  ld.lld: error: undefined symbol: __kcfi_typeid_clear_page_erms
  >>> referenced by ld-temp.o
  >>>               vmlinux.o:(__cfi_clear_page_erms)

With full LTO, it is possible for LLVM to realize that these functions never
have their address taken (as they are only used within an alternative, which
will make them a direct call) across the whole kernel and either drop or skip
generating their kCFI type identification symbols.

clear_page_{rep,orig,erms}() are defined in clear_page_64.S with
SYM_TYPED_FUNC_START as a result of

  2981557c ("x86,kcfi: Fix EXPORT_SYMBOL vs kCFI"),

as exported functions are free to be called indirectly thus need kCFI type
identifiers.

Use KCFI_REFERENCE with these clear_page() functions to force LLVM to see
these functions as address-taken and generate then keep the kCFI type
identifiers.

Fixes: 2981557c ("x86,kcfi: Fix EXPORT_SYMBOL vs kCFI")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2128


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://patch.msgid.link/20251013-x86-fix-clear_page-cfi-full-lto-errors-v1-1-d69534c0be61@kernel.org