Add "CHECK-" prefix to un-break buildbot failure.

After r188221, the prefix given by --check-prefix must match with the label
as a whole. Substring is not considered to be a correct label anymore.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@188227 91177308-0d34-0410-b5e6-96231b3b80d8