reproducer wiki should have an example on how to add cmake arguments

[caojoshua]

Failed LLVM buildbot jobs will link to https://github.com/google/sanitizers/wiki/SanitizerBotReproduceBuild. I found by looking at the scripts that we can add extra CMake arguments with CMAKE_COMMON_OPTIONS. This is really helpful for building locally and should be included in that wiki. For example,

CMAKE_COMMON_OPTIONS="-DLLVM_PARALLEL_LINK_JOBS=2" llvm-zorg/zorg/buildbot/builders/sanitizers/buildbot_fast.sh

is helpful for less powerful machines.