Makefile: fix build of io_uring on sh4

Makefile: fix build of io_uring on sh4

SuperH compile currently fails with:

/usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sh4-buildroot-linux-uclibc/8.3.0/../../../../sh4-buildroot-linux-uclibc/bin/ld: t/io_uring.o: in function `submitter_fn':
/usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-0/output-1/build/fio-3.19/t/io_uring.c:131: undefined reference to `arch_flags'
/usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sh4-buildroot-linux-uclibc/8.3.0/../../../../sh4-buildroot-linux-uclibc/bin/ld: /usr/lfs/hdd_v1/rc-buildroot-test/scripts/instance-0/output-1/build/fio-3.19/t/io_uring.c:367: undefined reference to `arch_flags'
collect2: error: ld returned 1 exit status

Fix that by ensuring we have a stub arch.o with the necessary arch flags

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>