[AIX][XCOFF] print unsupported message for llvm-ar big archive write operation

Summary:

when run "llvm-ar cr" on AIX OS , it created a gnu archive, it is not desirable in aix OS.
instead of creating a gnu archive, the patch will print a unsupport message for llvm-ar big archive write operation in AIX OS.

after implement the big archive operation, I will revert the XFAIL: AIX " and "--format=gnu" test cases in the patch.

Reviewer : James Henderson, Jinsong Ji
Differential Revision: https://reviews.llvm.org/D122746