[S390] __div64_32 for 31 bit.

[S390] __div64_32 for 31 bit.

The clocksource infrastructure introduced with commit
6d574399dc74e6b6f8181f6c1d378ae1750a0803 broke 31 bit s390.
The reason is that the do_div() primitive for 31 bit always
had a restriction: it could only divide an unsigned 64 bit
integer by an unsigned 31 bit integer. The clocksource code
now uses do_div() with a base value that has the most
significant bit set. The result is that clock->cycle_interval
has a funny value which causes the linux time to jump around
like mad.
The solution is "obvious": implement a proper __div64_32
function for 31 bit s390.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>