Enforce DWARF v5

When compiling with `-g`, this should be forwarded as `-gdwarf-5` to clang. DWARF v5 is necessary for generating valid debug information: The wasm specifics of the symbols are based on https://yurydelendik.github.io/webassembly-dwarf/, which is an extension to the DWARF v5 standard. Location expressions for wasm values as defined in that extension are not valid in DWARF v4 because they need to be typed, otherwise expressions would be limited to 32bit (in wasm32) which couldn't support 64bit integers or floats, or simd values.