[OpenMP][AIX] Implement __kmp_get_load_balance() for AIX (#91520)

AIX has the `/proc` filesystem where `/proc/<pid>/lwp/<tid>/lwpsinfo` has
the thread state in binary, similar to Linux's
`/proc/<pid>/task/<tid>/stat` where the state is in ASCII. However, the
definition of state info `R` in `lwpsinfo` is `runnable`. In Linux,
state `R` means the thread is `running`. Therefore, `lwpsinfo` is not
ideal for our purpose of getting the current load of the system. This
patch uses `perfstat_cpu()` in AIX system library `libperfstat.a` to
obtain the number of threads current running on logical CPUs.