[llvm-mc-fuzzer] Fix -jobs option.

[llvm-mc-fuzzer] Fix -jobs option.

The fuzzer argument parser will ignore all options starting with '--' so
operation mode options should begin with '--' and fuzzer options should begin
with '-'. Fuzzer arguments must still follow --fuzzer-args so that they escape
the parsing performed by the CommandLine library.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248671 91177308-0d34-0410-b5e6-96231b3b80d8