[AIX][XCOFF] Fix XCOFFObjectWriter assertion failure with alignment-related...

[AIX][XCOFF] Fix XCOFFObjectWriter assertion failure with alignment-related gap and improve text section output testing

SUMMARY:
1.if there is a gap between the end virtual address of one section and the beginning virtual address of the next section, the XCOFFObjectWriter.cpp will hit a assert.

2.as discussed in the patch https://reviews.llvm.org/D66969,
since implemented the function description. We can output the raw object data for function.
we need to create a test for raw text section content and test section header for xcoff object file.

Reviewer: daltenty,hubert.reinterpretcast,jasonliu

Differential Revision: https://reviews.llvm.org/D71845