[openmp][AIX]Initial changes for porting to AIX (#76841)

This PR contains initial changes for building and testing libomp on AIX.
More changes will follow.
- `KMP_OS_AIX` is defined for the AIX platform
- `KMP_ARCH_PPC` is defined for 32-bit PPC
- `KMP_ARCH_PPC_XCOFF` and `KMP_ARCH_PPC64_XCOFF` are for 32- and 64-bit
XCOFF object formats respectively
- Assembly file `z_AIX_asm.S` is used for AIX specific assembly code and
will be added in a separate PR
- The target library is disabled because AIX does not have the device
support
- OMPT is temporarily disabled