avoid linking using --export-all for webassembly #2290
Description
Currently Zig uses --export-all to the linker line when targeting WebAssembly. This prevents symbols from being deleted from compiler_rt.a and builtin.a. Instead, only functions explicitly marked export in the root source file should be exported. We should be able to create a truly minimal webassembly file if there are no dependencies on compiler_rt functions or builtin.a functions.