implement get_cpu_clock() for s390/s390x

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Move 'tsc_reliable' outside of ARCH_HAVE_CPU_CLOCK

Otherwise we fail building on architectures that do not define
it, as reported by Dan:

cc -o gettime.o -c -std=gnu99 -Wwrite-strings -Wall -O3 -g -O2 -g -pipe
-Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector
--param=ssp-buffer-size=4 -m31 -march=z9-109 -mtune=z10
-DFIO_VERSION='"fio-2.0.12.2"' -D_GNU_SOURCE -D_LARGEFILE_SOURCE
-D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -DFIO_INC_DEBUG gettime.c
gettime.c: In function 'fio_clock_init':
gettime.c:317:6: error: 'tsc_reliable' undeclared (first use in this function)
gettime.c:317:6: note: each undeclared identifier is reported only once for each function it appears in
make: *** [gettime.o] Error 1

Reported-by: Dan Horák <dan@danny.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>