[XCOFF] Enable -fdata-sections on AIX

Summary:
Some design decision worth noting about:

I've noticed a recent mailing discussing about why string literal is
not affected by -fdata-sections for ELF target:
http://lists.llvm.org/pipermail/llvm-dev/2020-September/145121.html

But on AIX, our linker could not split the mergeable string like other target.
So I think it would make more sense for us to emit separate csect for
every mergeable string in -fdata-sections mode,
as there might not be other ways for linker to do garbage collection
on unused mergeable string.

Reviewed By: daltenty, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D88339