[CMake] Enable -qfuncsect when building with IBM XL

Summary:
The IBM XL compiler uses `-qfuncsect` for `-ffunction-sections`.

The comment about sanitizers and `-f[no-]function-sections` is corrected
also, as it is pertinent to this patch.

The sanitizer-related use of `-fno-function-sections` is associated with
powerpc64le, a target for which there is an IBM XL compiler, so that use
is updated in this patch to apply `-qnofuncsect` in case a build using
the XL compiler is viable on that platform.

This patch has been verified with the XL compiler on AIX only.

Reviewers: daltenty, stevewan

Reviewed By: daltenty

Subscribers: mgorny, steven.zhang, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D72335