[AIX] fixed llvm-ar can not read empty big archive correctly.

Summary:

llvm-ar can not read empty big archive correctly. it output error as
error: unable to load 'empty.a': truncated or malformed archive (characters in size field in archive member header are not all decimal numbers: '<bigaf>'

Reviewers: James Henderson
Differential Revision: https://reviews.llvm.org/D124017