omp_locks: Avoid extern global variable (#3798)

Make `omp_locks` a variable in an unnamed namespace instead of an extern
global variable.

This might potentially fix the Windows symbol issue (#3795).

Src/Base/AMReX_BaseFab.H

@@ -3360,9 +3360,7 @@ BaseFab<T>::lockAdd (const BaseFab<T>& src, const Box& srcbox, const Box& destbo
         while (planes_left > 0) {
             AMREX_ASSERT(mm < nplanes);
             auto const m = mm + plo;
-            int ilock = m % OpenMP::nlocks;
-            if (ilock < 0) { ilock += OpenMP::nlocks; }
-            auto* lock = &(OpenMP::omp_locks[ilock]);
+            auto* lock = OpenMP::get_lock(m);
             if (omp_test_lock(lock))
             {
                 auto lo = dlo;

Src/Base/AMReX_OpenMP.H

@@ -17,8 +17,7 @@ namespace amrex::OpenMP {
     void Initialize ();
     void Finalize ();
 
-    static constexpr int nlocks = 128;
-    extern AMREX_EXPORT omp_lock_t omp_locks[nlocks];
+    omp_lock_t* get_lock (int ilock);
 }
 
 #else // AMREX_USE_OMP

Src/Base/AMReX_OpenMP.cpp

@@ -135,9 +135,9 @@ namespace amrex
 #ifdef AMREX_USE_OMP
 namespace amrex::OpenMP
 {
-    omp_lock_t omp_locks[nlocks];
-
     namespace {
+        constexpr int nlocks = 128;
+        omp_lock_t omp_locks[nlocks];
         unsigned int initialized = 0;
     }
 
@@ -204,5 +204,12 @@ namespace amrex::OpenMP
         }
     }
 
+    omp_lock_t* get_lock (int ilock)
+    {
+        ilock = ilock % nlocks;
+        if (ilock < 0) { ilock += nlocks; }
+        return omp_locks + ilock;
+    }
+
 } // namespace amrex::OpenMP
 #endif // AMREX_USE_OMP