[compiler-rt] libFuzzer: update -merge_control_file= help message.

Summary:
The motivation for this change is to have a distinguisher in libFuzzer
that would let the runner know whether multistep merge is supported or not by
a particular fuzz target binary. Otherwise, multistep merge fails to execute
with older version of libFuzzer, and there is no way to verify that easily.

Reviewers: kcc

Subscribers: dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71423