[OpenMP][AIX] Set worker stack size to 2 x KMP_DEFAULT_STKSIZE if system stack...

[OpenMP][AIX] Set worker stack size to 2 x KMP_DEFAULT_STKSIZE if system stack size is too big (#81996)

This patch sets the stack size of worker threads to `2 x
KMP_DEFAULT_STKSIZE` (2 x 4MB) for AIX if the system stack size is too
big. Also defines maximum stack size for 32-bit AIX.