contrib: aribb24: add -pthread for EH/SjLj use in libpng - wasm-emscripten

For the webassembly compiler, the -pthread option will enable the +atomics feature to prevent modules that use shared memory to mix atomic operations with non-atomic ones. (cf. https://github.com/WebAssembly/tool-conventions/blob/main/Linking.md#target-features-section)

A recent introduction in the compiler code requires atomics to be enabled if EH/SjLj is used. (cf. https://reviews.llvm.org/D120013)

Since the libpng module will end up having +atomics in it, and the application based on libvlc will have it too - Not having it in the dependencies or in the libraries using libpng will cause a linking error.

This commit adds the +atomics feature in libpng, zlib and aribb24