InstrProf: Change magic number to have non-text characters

Include non-text characters in the magic number so that text files can't
match.

<rdar://problem/15950346>

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@204514 91177308-0d34-0410-b5e6-96231b3b80d8