[sanitizer] Re-enable -fno-function-sections for powerpc64le

Summary:
For a reason that hasn't been investigated for lack of powerpc knowledge and
hardware, -fno-function-sections is required for the Sanitizers to work
properly on powerpc64le. Without, the function-sections-are-bad test fails on
that architecture (and that architecture only).

This patch re-enables the flag in the powerpc64le cflags.

I have to admit I am not entirely sure if my way is the proper way to do this,
so if anyone has a better way, I'll be happy to oblige.

Reviewers: kcc, eugenis

Reviewed By: eugenis

Subscribers: nemanjai, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D29285

llvm-svn: 293558