[llvm-ar] Add object mode option -X for AIX

Summary:

1. Added a new option object mode -X for llvm-ar. In AIX OS , there is a object mode option -X for ar command.
please see the "-X mode" part of https://www.ibm.com/docs/ko/aix/7.1?topic=ar-command

Specifies the type of object file ar should examine. The mode must be one of the following:
32
Processes only 32-bit object files
64
Processes only 64-bit object files
32_64
Processes both 32-bit and 64-bit object files
any
Processes all of the supported object files.

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 ar to process any 64-bit objects and ignore 32-bit objects. The -X flag overrides the OBJECT_MODE variable.

2. Before adding the new option -X, the default behaviors of llvm-ar like -Xany, but after the adding the new option -X, the default behaviors of llvm-ar change to -X32 ,in order to let some test cases which has 32bit and 64bit object file in the same llvm-ar command, we need to add the "export OBJECT_MODE=any" into test case to change the default behaviors of llvm-ar's object mode.

Reviewers: James Henderson, Owen Reynolds, Fangrui Song
Differential Revision: https://reviews.llvm.org/D127864