fix rlock compilation failure (#15584)

* change SysLockType_Reentrant

fix edge case where using SysLockType_Reentrant doesn't trigger an #include pthread.h

* syslocktype_reentrant now a var
* remove nodecl to remove empty system_syslocks.c
* let is better than var.

in reality SysLockType = enum, maybe that would be a better fix

lib/core/rlocks.nim

@@ -24,7 +24,7 @@ proc initRLock*(lock: var RLock) {.inline.} =
   when defined(posix):
     var a: SysLockAttr
     initSysLockAttr(a)
-    setSysLockType(a, SysLockType_Reentrant())
+    setSysLockType(a, SysLockType_Reentrant)
     initSysLock(lock, a.addr)
   else:
     initSysLock(lock)

lib/system/syslocks.nim

@@ -181,8 +181,8 @@ else:
       releaseSysAux(L)
 
   when insideRLocksModule:
-    proc SysLockType_Reentrant: SysLockType =
-      {.emit: "`result` = PTHREAD_MUTEX_RECURSIVE;".}
+    let SysLockType_Reentrant{.importc: "PTHREAD_MUTEX_RECURSIVE",
+      header: "<pthread.h>".}: SysLockType
     proc initSysLockAttr(a: var SysLockAttr) {.
       importc: "pthread_mutexattr_init", header: "<pthread.h>", noSideEffect.}
     proc setSysLockType(a: var SysLockAttr, t: SysLockType) {.