[OpenMP][AIX] Fix test config for AIX (#88272)

This patch fixes the test config so that it works for
`tasking/omp50_taskdep_depobj.c` which uses different flags to test with
compiler's `omp.h`.
* set test environment variable `OBJECT_MODE` to `64` if it is set
explicitly to `64` in the AIX environment. `OBJECT_MODE` is default to
`32` and is recognized by AIX compilers and toolchain. In this way, we
don't need to set `-m64` for all compiler flags for 64-bit mode
* add option `-Wl,-bmaxdata` to 32-bit `test_openmp_flags` used by
`tasking/omp50_taskdep_depobj.c`