unify wasm options

[DerThorsten]

See jupyter-xeus/xeus-lite#7 for details, but in a nutshell, we want all xeus-wasm kernels to be as similar / unified as we can. Therefore we:

• drop the async option from the wasm options. If a kernel really wants to be async, the flag can be set in the kernel directly.
• force all kernels to use dynamic linking (even though atm this is only used by python, most kernels should use that, in particular lua and any upcoming R kernel)

In this PR we:

• drop the async option function
• added the needed flags for dynamic linking (MAIN_MODULE)
• removed the filesystem option and just force a filesystem for all kernels
• add POSITION_INDEPENDENT_CODE flag for all kernels, ie -fPIC, since this is mandatory for all used libraries when using dynamic linking.
• remove superfluous flag XEUS_EMSCRIPTEN_WASM_BUILD to indicate emscripten build, we can just use EMSCRIPTEN which already does the job.