Use linker tools on OSX & Linux to limit exports (#4651)

[steven-johnson]

As a way to address #4651, but as a hopefully-simpler alternative to #5655, this adds linker-specific scripts for Linux and OSX builds that exports only the Halide:: and halide_ symbols in our shared libraries, hiding everything else (most notable, everything LLVM).

It doesn't attempt to do the same for MSVC builds (yet). Even without that, though, this is arguably strictly better than what we had before.

Marking as "draft" for now because the CMake scripting is awful but I didn't want to wait to figure out the right CMake syntax before trying this out. And also, some ugly TODOs and stuff.

[steven-johnson]

Feel free to push your corrections directly to this PR

[alexreinking]

Feel free to push your corrections directly to this PR

That's the plan 🙂

[abadams]

The linker scripts are much less arcane than I feared

[alexreinking]

Okay, I have tested my CMake changes on Linux with GCC and the following linkers: GNU LD, GOLD (via -fuse-ld=gold), and LLD (via -fuse-ld=lld). Changing the linker is a toolchain-level change and so requires a clean rebuild to re-run the check.

[steven-johnson]

Looks good! Just needs reviewer approval.

[LebedevRI]

The caveat with this approach alone as opposed to #5655 is that while it resolves the issue for the final library,
during the compile time however, the functions aren't known to be hidden, which may be suboptimal.

[alexreinking]

@LebedevRI - IIRC the issue was that we consume standard library types through our API which makes the situation on Windows a lot harder. For instance, I don't think you can export a class that takes in a std::vector.

[LebedevRI]

@LebedevRI - IIRC the issue was that we consume standard library types through our API which makes the situation on Windows a lot harder. For instance, I don't think you can export a class that takes in a std::vector.

I don't know the windows situation, but i would like to note that even with the current linker script approach,
it is not done for windows, only *nix-like. So i'm a little unsure why an inability to do so on one platform
is a blocker for doing so for the other platforms?