[AIX] Define __HOS_AIX__ macro only for AIX target

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This patch defines the macro __HOS_AIX__ when the target is AIX and without any dependency on the host. The macro indicates that the host is AIX. Defining the macro will help minimize porting pain for existing code compiled with xlc/xlC. xlC never shipped cross-compiling support, so the difference is not observable anyway.
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This is a follow up to the discussion in https://reviews.llvm.org/D107242.

Reviewed By: cebowleratibm, joerg

Differential Revision: https://reviews.llvm.org/D107825