[CMake][AIX] Limit tools in external project build

This is a follow on to D85329 which disabled some llvm tools in the
runtimes build due to XCOFF64 limitations. This change disables them
in other external project builds as well, when no list of tools is
specified in the arguments.

Reviewed By: hubert.reinterpretcast, stevewan

Differential Revision: https://reviews.llvm.org/D88310