microblaze: generic atomic64 support

This tiny patch adds generic atomic64 support for the Microblaze
architecture.

The patch is against the latest linux-2.6-microblaze tree. It also
fixes the kernel build for microblaze:

Error log:
CC kernel/trace/trace_clock.o
kernel/trace/trace_clock.c:117: error: expected '=', ',', ';', 'asm'
or '__attribute__' before 'trace_counter'
kernel/trace/trace_clock.c: In function 'trace_clock_counter':
kernel/trace/trace_clock.c:126: error: implicit declaration of
function 'atomic64_add_return'
kernel/trace/trace_clock.c:126: error: 'trace_counter' undeclared
(first use in this function)
kernel/trace/trace_clock.c:126: error: (Each undeclared identifier is
reported only once
kernel/trace/trace_clock.c:126: error: for each function it appears in.)
make[2]: *** [kernel/trace/trace_clock.o] Error 1
make[1]: *** [kernel/trace] Error 2
make: *** [kernel] Error 2

Signed-off-by: Ariane Keller <ariane.keller@tik.ee.ethz.ch>
Signed-off-by: Daniel Borkmann <daniel.borkmann@tik.ee.ethz.ch>
Signed-off-by: Michal Simek <monstr@monstr.eu>