um: Add cmpxchg8b_emu and checksum functions to asm-prototypes.h

um: Add cmpxchg8b_emu and checksum functions to asm-prototypes.h

With CONFIG_GENDWARFKSYMS, um builds fail due to missing prototypes
in asm/asm-prototypes.h. Add declarations for cmpxchg8b_emu and the
exported checksum functions, including csum_partial_copy_generic as
it's also exported.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503251216.lE4t9Ikj-lkp@intel.com/
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://patch.msgid.link/20250326190500.847236-2-samitolvanen@google.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>