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 <[e-mail]>
Cc: [e-mail]
Reported-by: kernel test robot <[e-mail]>
Closes: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Sami Tolvanen <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/include/asm/asm-prototypes.h

···
 #include <asm-generic/asm-prototypes.h>
+#include <asm/checksum.h>
+
+#ifdef CONFIG_UML_X86
+extern void cmpxchg8b_emu(void);
+#endif

arch/x86/um/asm/checksum.h

···
  */
 extern __wsum csum_partial(const void *buff, int len, __wsum sum);
 
+/* Do not call this directly. Declared for export type visibility. */
+extern __visible __wsum csum_partial_copy_generic(const void *src, void *dst, int len);
+
 /**
  * csum_fold - Fold and invert a 32bit checksum.
  * sum: 32bit unfolded sum