patch 9.1.0630: MS-Windows: build fails with VIMDLL and mzscheme

patch 9.1.0630: MS-Windows: build fails with VIMDLL and mzscheme

Problem:  MS-Windows: build fails with VIMDLL and mzscheme
Solution: define scheme_register_tls_space() inside gvim.exe
          and refer to it from the dll (Ken Takata).

`scheme_register_tls_space()` doesn't support a thread-local variable in
a DLL:
https://docs.racket-lang.org/inside/im_memoryalloc.html#%28cpp._scheme_register_tls_space%29

closes: #15363

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>