JS: Improve compatibility with recent emsdk

The JavaScript code in libraries/base/jsbits/base.js had some
hardcoded offsets for fields in structs, because we expected
the layout of the data structures to remain unchanged. Emsdk
3.1.42 changed the layout of the stat struct, breaking this
assumption, and causing code in .hsc files accessing the
stat struct to fail.

This patch improves compatibility with recent emsdk by
removing the assumption that data layouts stay unchanged:

    1. offsets of fields in structs used by JavaScript code are
       now computed by the configure script, so both the .js and
       .hsc files will automatically use the new layout if anything
       changes.
    2. the distrib/configure script checks that the emsdk version
       on a user's system is the same version that a bindist was
       booted with, to avoid data layout inconsistencies

See #23641

(cherry picked from commit aa07402e)