[AIX] Try to not use LLVM tools while building runtimes

Since 64-bit XCOFF and the big AR format is not yet supported in some of these tools, this patch avoids additional setup of these tools. This patch is not intended to prevent picking up the LLVM tools if they happen to be available otherwise.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D85329