Fix test issue with gcc-11
    
    Test 3.t exercises io_submit() with invalid addresses,
    and one test attempted to pass in "-1" as an invalid
    pointer. But gcc-11 realizes you cannot take an offset
    from an invalid pointer, making it impossible to
    compile or run this test:
    
    > sh# make CC=gcc-11 partcheck
    > make[1]: Entering directory '/alt/public_software/libaio/src'
    > make[1]: Nothing to be done for 'all'.
    > make[1]: Leaving directory '/alt/public_software/libaio/src'
    > make[1]: Entering directory '/alt/public_software/libaio/harness'
    > gcc-11 -Wall -Werror -I../src -g -O2 -DTEST_NAME=\"cases/2.t\" -o cases/2.p main.c ../src/libaio.a -lpthread
    > gcc-11 -Wall -Werror -I../src -g -O2 -DTEST_NAME=\"cases/3.t\" -o cases/3.p main.c ../src/libaio.a -lpthread
    > In file included from main.c:24:
    > cases/3.t: In function ‘test_main’:
    > cases/3.t:18:19: error: ‘attempt_io_submit’ accessing 8 bytes in a region of size 0 [-Werror=stringop-overflow=]
    >    18 |         status |= attempt_io_submit( io_ctx, 1, (void *)-1, -EFAULT);
    >       |                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > cases/3.t:18:19: note: referencing argument 3 of type ‘struct iocb **’
    > In file included from cases/3.t:5,
    >                  from main.c:24:
    > cases/aio_setup.h:15:5: note: in a call to function ‘attempt_io_submit’
    >    15 | int attempt_io_submit(io_context_t ctx, long nr, struct iocb *ios[], int expect)
    >       |     ^~~~~~~~~~~~~~~~~
    > cc1: all warnings being treated as errors
    > make[1]: *** [Makefile:19: cases/3.p] Error 1
    > make[1]: Leaving directory '/alt/public_software/libaio/harness'
    > make: *** [Makefile:23: partcheck] Error 2
    
    The fix is to tell gcc to relax it's stringop overflow
    testing for this one test.
    
    Changes since v1:
    * First version just skipped the test in question
    
    Signed-off-by: Lee Duncan <leeman.duncan@gmail.com>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>