[win32] Various changes to make it build cleanly and pass all tests:

[win32] Various changes to make it build cleanly and pass all tests:
 - needed to run `perl embed.pl`
 - use PERL_CORE instead of PERLDLL in places that do mean PERL_CORE
 - fix prototypes for a few declarations (Borland is finally quiet)
 - move declaration of Mymalloc etc to perl.h (since win32 and other
   ports may #define malloc themselves, to let extensions bind to
   the version that perl used)
 - move struct reg_data into a public header file, since it is
   referenced in a public datatype
 - win32 makefile fixes
 - fix remaining s/thread/perl_thread/

p4raw-id: //depot/win32/perl@304