[AIX]Lowering global address for 32/64bit small/large code models

[AIX]Lowering global address for 32/64bit small/large code models

    This patch implements global address lowering for 32/64 bit with small/large code models.
    1.For 32bit large code model on AIX, there are newly added pseudo opcode LWZtocL & ADDIStocHA32, the support of which on MC layer will be
       provided by future patches.
    2.The default code model on AIX should be small code model.
    3.Since AIX does not have medium code model, "report_fatal_error" when users specify it.

    Differential Revision: https://reviews.llvm.org/D63547

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368744 91177308-0d34-0410-b5e6-96231b3b80d8