Fix code section prefix for proper layout

Summary:
r284533 added hot and cold section prefixes based on profile
information, to enable grouping of hot/cold functions at link time.
However, it used "cold" as the prefix for cold sections, but gold only
recognizes "unlikely" (which is used by gcc for cold sections).
Therefore, cold sections were not properly being grouped. Switch to
using "unlikely"

Reviewers: danielcdh, davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32983

llvm-svn: 302502