* cygwin.din (_impure_ptr): Export.

(reent_data): Don't export.
* dcrt0.cc (reent_data): Drop.
(__cygwin_user_data): Initialize impure_ptr as GLOBAL_REENT.
(_dll_crt0): Initialize _impure_ptr as GLOBAL_REENT.
* lib/_cygwin_crt0_common.cc (_impure_ptr): Drop.  Declare extern as dllimport
instead.

winsup/cygwin/cygwin.din

@@ -12,6 +12,7 @@ __rcmd_errstr DATA
 _check_for_executable DATA
 _ctype_ DATA
 _daylight DATA
+_impure_ptr DATA
 _sys_errlist DATA
 sys_errlist = _sys_errlist DATA
 _sys_nerr DATA
@@ -24,7 +25,6 @@ opterr DATA
 optind DATA
 optopt DATA
 optreset DATA
-reent_data DATA
 _alloca NOSIGFE
 dll_entry@12 NOSIGFE
 __assert SIGFE