Since commit 0f150b69d3 (AIX: Explicitly compute shared object exports
for both XL and GNU, 2019-07-11, v3.16.0-rc1~418^2~2) we always export
all symbols from shared libraries by default. Add a new target property
called `AIX_EXPORT_ALL_SYMBOLS` that can be explicitly set to OFF to
suppress this behavior and export no symbols by default.
Fixes: #20290
