Fix compile of test programs on archs that use arch_flags at runtime

Fix compile of test programs on archs that use arch_flags at runtime

SuperH compile currently fails with:

gettime.o: In function fio_gettime':
/home/thomas/projets/buildroot/output/build/fio-fio-2.7/gettime.c:163: undefined reference to arch_flags'
gettime.o: In function utime_since_now':
/home/thomas/projets/buildroot/output/build/fio-fio-2.7/gettime.c:164: undefined reference to arch_flags'
gettime.o: In function mtime_since_now':
/home/thomas/projets/buildroot/output/build/fio-fio-2.7/gettime.c:164: undefined reference to arch_flags'
gettime.o: In function time_since_now':
/home/thomas/projets/buildroot/output/build/fio-fio-2.7/gettime.c:164: undefined reference to arch_flags'
mutex.o: In function fio_mutex_up':
/home/thomas/projets/buildroot/output/build/fio-fio-2.7/mutex.c:189: undefined reference to arch_flags'
collect2: error: ld returned 1 exit status
Makefile:375: recipe for target 't/stest' failed
make[2]: *** [t/stest] Error 1
make[2]: *** Waiting for unfinished jobs....

Fix that by ensuring we have a stub arch.o with the necessary arch flags
for the standalone test programs.

Signed-off-by: Jens Axboe <axboe@fb.com>