lib/checksum.c: make do_csum optional

Mike Frysinger suggested that do_csum should be optional
so that an architecture can use the generic checksum code
but still provide an optimized fast-path for the most
critical function.

This can mean an implementation using inline assembly,
or in case of Alpha one using 64-bit arithmetic in C.

Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>