[OpenMP][AIX] Use syssmt() to get the number of SMTs per physical CPU (#89985)

This patch changes to use system call `syssmt()` instead of
`lpar_get_info()` to get the number of SMTs (logical processors) per
physical processor for AIX. `lpar_get_info()` gives the max number of
SMTs that the physical processor can support while `syssmt()` returns
the number that is currently configured.