[XCOFF][AIX] Put undefined symbol name into StringTable when neccessary

Summary:
When we have a long name for the undefined symbol, we would hit this assertion:
Assertion failed: I != StringIndexMap.end() && "String is not in table!"
This patch addresses that.

Reviewed by: DiggerLin, daltenty

Differential Revision: https://reviews.llvm.org/D74924