[AIX] Limit `extract_symbols.py`'s Pools Size to 8

On AIX, `extract_symbols.py`'s default pool size may be too big for the logical partition(LPAR)'s allocated memory, and may lead to intermittent IO errors. This patch limits the pool size to 8 to avoid such errors.

Reviewed By: daltenty, thieta

Differential Revision: https://reviews.llvm.org/D157884