Remove configure probe for dlopen, and refactor.

dlopen() is in SUSv2 and all targeted Unix systems have it.  We still
need replacement functions for Windows, but we don't need a configure
probe for that.

Since it's no longer needed by other operating systems, rename dlopen.c
to win32dlopen.c and move the declarations into win32_port.h.

Likewise, the macros RTLD_NOW and RTLD_GLOBAL now only need to be
defined on Windows, since all targeted Unix systems have 'em.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/port/win32dlopen.c

@@ -1,22 +1,20 @@
 /*-------------------------------------------------------------------------
  *
- * dlopen.c
- *	  dynamic loader for platforms without dlopen()
+ * win32dlopen.c
+ *	  dynamic loader for Windows
  *
  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  *
  * IDENTIFICATION
- *	  src/port/dlopen.c
+ *	  src/port/win32dlopen.c
  *
  *-------------------------------------------------------------------------
  */
 
 #include "c.h"
 
-#if defined(WIN32)
-
 static char last_dyn_error[512];
 
 static void
@@ -93,5 +91,3 @@ dlopen(const char *file, int mode)
 	last_dyn_error[0] = 0;
 	return (void *) h;
 }
-
-#endif