[AIX] Update data directives for AIX assembly

Summary:
The standard data emission directives (e.g. .short, .long) in the AIX assembler
have the unintended consequence of aligning their output to the natural byte
boundary. This cause problems because we aren't expecting behavior from the
Data*bitsDirectives, so the final alignment of data isn't correct in some cases
on AIX.

This patch updated the Data*bitsDirectives to use .vbyte pseudo-ops instead to emit the
data, since we will emit the .align directives as needed. We update the existing
testcases and add a test for emission of struct data.

Reviewers: hubert.reinterpretcast, Xiangling_L, jasonliu

Reviewed By: hubert.reinterpretcast, jasonliu

Subscribers: wuzish, nemanjai, hiraditya, kbarton, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80934