Fix nim-lang#12135 and nim-lang#12109 (nim-lang#12137)

* Fix nim-lang#12135 and fix nim-lang#12109 - fix OpenBSD type defs

* Fix Mode definition as in nim-lang#12132

Author: euantorano

lib/posix/posix.nim

@@ -97,6 +97,8 @@ const StatHasNanoseconds* = defined(linux) or defined(freebsd) or
 
 when (defined(linux) and not defined(android)) and defined(amd64):
   include posix_linux_amd64
+elif defined(openbsd) and defined(amd64):
+  include posix_openbsd_amd64
 elif (defined(macos) or defined(macosx) or defined(bsd)) and defined(cpu64):
   include posix_macos_amd64
 elif defined(nintendoswitch):