[AIX] XFAIL CodeGen/Generic/externally_available.ll

    Globals with “available_externally” linkage should never be emitted into the
    object file corresponding to the LLVM module.

    However, AIX system assembler default print error for undefined reference .
    so AIX chose to emit the available externally symbols into .s,
    so that users won't run into errors in situations like:

    clang -target powerpc-ibm-aix -xc -<<<$'extern inline
    __attribute__((__gnu_inline__)) void foo() {}\nvoid bar() { foo(); }' -O
    -Xclang -disable-llvm-passes

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D102377