The new 'split_libmodules=yes' option is useful to work around linker
command line size limitations when linking a huge number of objects.
We're currently over 64k chars when linking libmodules.a on Windows
with MinGW, which triggers issues as seen in #30892.

Even on Linux, we can also reach linker command line size limitations
by adding more custom modules.

We force this option to True for MinGW on Windows, which fixes #30892.

Additional changes to lib splitting:

- Fix linking of the split module libs with interdependent symbols,
  hacking our way into LINKCOM and SHLINKCOM to set the `--start-group`
  and `--end-group` flags.
- Fix Python 3 compatibility in `methods.split_lib()`.
- Drop seemingly obsolete condition for 'msys' on 'posix'.
- Drop the unnecessary 'split_drivers' as the drivers lib is no longer
  too big since we moved all thirdparty builds to modules.

Co-authored-by: Hein-Pieter van Braam-Stewart <hp@tmm.cx>