JS: widen emsdk version compatibility

This patch improves support for a wider version range of emsdk, avoiding breakage if struct layouts change.

• This removes hardcoded offsets from h$base_fillStat in base/jsbits/base.js, relying instead of values computed by the C compiler and inserted into HsBaseConfig.h
• This adds a version check for emsdk, in particular ensuring that the emsdk version on a user system is the same as the version a bindist was booted with

Note: there is currently no upper bound on the emsdk version. If we get all the data layouts from the C compiler, future breakage should be unlikely (hopefully, haha). There are lots of emsdk releases and not allowing anything newer than the GHC release would be rather inconvenient for users.