[llvm-isel-fuzzer] Use "--" as separator rather than '='.

Summary: OSS-Fuzz doesn't support '=' in filenames.

Reviewers: bogner, kcc

Reviewed By: kcc

Subscribers: javed.absar, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D38866

llvm-svn: 315647