[AIX] Improve 32/64-bit build configuration

Summary:
AIX supports both 32-bit and 64-bit environments (with 32-bit being the default). This patch improves support for building LLVM on AIX in both 32-bit and 64-bit mode.

  - Change host detection to return correct 32/64-bit triple as config_guess does not return the correct version on 64-bit. This can confuse JIT tests and other things that care about what the host triple is.
  - Remove manual setting of 64-bit flags on AIX. AIX provides OBJECT_MODE environment variable to enable the user to obtain a 64-bit development environment. CMake will properly set these flags provided the user sets the correct OBJECT_MODE before configuring and setting them manually will interfere with 32-bit builds.
  - Don't present the LLVM_BUILD_32_BITS option on AIX, users should use OBJECT_MODE when running CMake instead.

Reviewers: hubert.reinterpretcast, DiggerLin, stevewan

Reviewed By: DiggerLin, stevewan

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74256