[AIX] make integrated-as as default on AIX.

Summary: Clang uses LLVM's integrated assembler by default on most targets, however non-integrated-as mode is default on AIX. Currently integrated-as mode on AIX has passed tests of LLVM test-suite, bootstrap and Spec2017, therefore this patch sets integrated-as as the default assembler mode on AIX.

Reviewed By: DiggerLin

Differential Revision: https://reviews.llvm.org/D150758