[AIX][XCOFF] Generate undefined symbol in symbol table for external function call

Summary:
This patch sets up the infrastructure for

 1. Associate MCSymbolXCOFF with an MCSectionXCOFF when it could not
    get implicitly associated.
 2. Generate undefined symbols. The patch itself generates undefined symbol
    for external function call only. Generate undefined symbol for external
    global variable and external function descriptors will be handled in
    separate patch(s) after this is land.

Differential Revision: https://reviews.llvm.org/D70443