Pass `OS=POSIX` env for macOS

[eirnym]

CodeWrangler is a closed source binary and has been last released in 2017 and it seems that it won't get any updates anytime soon. Thus honggfuzz will fail to link on macOS for aarch64 architecture.

However, there's (merged) implementation in honggfuzz which allows to build and work correctly on macOS without attaching CodeWrangler code at all, by passing OS=POSIX.

Could you please update the caller code to have an option to pass OS=POSIX on macOS at least for aarch64 architecture.

[PaulGrandperrin]

Thanks for the info, I don't have an macOS machine with me right now but I should be able to try that soon and if everything works fine, I'll release a new version with it.

[eirnym]

Meanwhile, there's few issues and PRs you can read in the original repo to fill the gap.

I'd make this as a macOS-specific feature enabled by default for aarch64 but available for x86_64, as system-dependent (if it works well on modern macOS anyway).