lib/crc64: add support for arch-optimized implementations

Add support for architecture-optimized implementations of the CRC64
library functions, following the approach taken for the CRC32 and
CRC-T10DIF library functions.

Also take the opportunity to tweak the function prototypes:
- Use 'const void *' for the lib entry points (since this is easier for
  users) but 'const u8 *' for the underlying arch and generic functions
  (since this is easier for the implementations of these functions).
- Don't bother with __pure.  It's an unusual optimization that doesn't
  help properly written code.  It's a weird quirk we can do without.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Acked-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250130035130.180676-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

lib/Kconfig

@@ -201,6 +201,13 @@ config CRC64
 	  the kernel tree does. Such modules that use library CRC64
 	  functions require M here.
 
+config ARCH_HAS_CRC64
+	bool
+
+config CRC64_ARCH
+	tristate
+	default CRC64 if ARCH_HAS_CRC64 && CRC_OPTIMIZATIONS
+
 config CRC4
 	tristate "CRC4 functions"
 	help