Update cmake toolchain file to use _INIT variable for cflags and cxxflags as well

[ryanking13]

Follow up of #246

[ryanking13]

It seems like there are some libraries that overwrites the CMAKE_C_FLAGS_INIT when CMAKE_C_FLAGS are passed. I am not sure what the best convention is for this, but at least this breaks some existing recipes in pyodide-recipes.

[Andrej730]

Any particular packages that break with this? Wanted to try to look into it.

[ryanking13]

@Andrej730 I found that it breaks some libraries that we pass custom CMAKE_C_FLAGS (e.g. libzfp). If we change to _INIT variable, setting custom CMAKE_C_FLAGS will overwrite our default flags. Even when users pass custom CMAKE_C_FLAGS we want our default cmake flags prepended as they need to be always present.

I am not sure what is the best way to achieve this though.