[AIX] Use .space instead of .zero in assembly

Summary:
The AIX system assembler does not understand .zero, so we should prefer
emitting .space.

Subscribers: nemanjai, hiraditya, kbarton, MaskRay, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68815

llvm-svn: 374564