[AIX] llvm-nm support environment "OBJECT_MODE" for option -X on AIX OS

Summary:

according nm in AIX OS , https://www.ibm.com/docs/en/aix/7.2?topic=n-nm-command

In AIX OS, The default is to process 32-bit object files (ignore 64-bit objects). The mode can also be set with the OBJECT_MODE environment variable. For example, OBJECT_MODE=64 causes nm to process any 64-bit objects and ignore 32-bit objects. The -X flag overrides the OBJECT_MODE variable.

In non AIX OS. The default is to process all support object files. and not support the OBJECT_MODE environment variable.

Reviewers:  James Henderson

Differential Revision: https://reviews.llvm.org/D132494