Fix Win arm GC barrier stuff

My recent change to enable using LLD on Unix inadvertedly broken
Windows ARM GC barriers. This change makes part of that change
Unix specific.

src/coreclr/vm/arm/stubs.cpp

@@ -281,8 +281,13 @@ void StubLinkerCPU::Init(void)
 // value of the global into a register.
 struct WriteBarrierDescriptor
 {
-    DWORD   m_pFuncStart;                   // Offset to the start of the barrier function relative to this struct address
-    DWORD   m_pFuncEnd;                     // Offset to the end of the barrier function relative to this struct address
+#ifdef TARGET_UNIX
+    DWORD   m_funcStartOffset;              // Offset to the start of the barrier function relative to this struct address
+    DWORD   m_funcEndOffset;                // Offset to the end of the barrier function relative to this struct address
+#else // TARGET_UNIX
+    BYTE *  m_pFuncStart;                   // Pointer to the start of the barrier function
+    BYTE *  m_pFuncEnd;                     // Pointer to the end of the barrier function
+#endif // TARGET_UNIX
     DWORD   m_dw_g_lowest_address_offset;   // Offset of the instruction reading g_lowest_address
     DWORD   m_dw_g_highest_address_offset;  // Offset of the instruction reading g_highest_address
     DWORD   m_dw_g_ephemeral_low_offset;    // Offset of the instruction reading g_ephemeral_low
@@ -436,18 +441,28 @@ void UpdateGCWriteBarriers(bool postGrow = false)
     // Iterate through the write barrier patch table created in the .clrwb section
     // (see write barrier asm code)
     WriteBarrierDescriptor * pDesc = &g_rgWriteBarrierDescriptors;
+#ifdef TARGET_UNIX
+    while (pDesc->m_funcStartOffset)
+#else // TARGET_UNIX
     while (pDesc->m_pFuncStart)
+#endif // TARGET_UNIX
     {
         // If the write barrier is being currently used (as in copied over to the patchable site)
         // then read the patch location from the table and use the offset to patch the target asm code
-        PBYTE to = FindWBMapping((BYTE *)pDesc + pDesc->m_pFuncStart);
+#ifdef TARGET_UNIX
+        PBYTE to = FindWBMapping((BYTE *)pDesc + pDesc->m_funcStartOffset);
+        size_t barrierSize = pDesc->m_funcEndOffset - pDesc->m_funcStartOffset;
+#else // TARGET_UNIX
+        PBYTE to = FindWBMapping(pDesc->m_pFuncStart);
+        size_t barrierSize = pDesc->m_pFuncEnd - pDesc->m_pFuncStart;
+#endif // TARGET_UNIX
         if(to)
         {
             to = (PBYTE)PCODEToPINSTR((PCODE)GetWriteBarrierCodeLocation(to));
             ExecutableWriterHolder<BYTE> barrierWriterHolder;
             if (IsWriteBarrierCopyEnabled())
             {
-                barrierWriterHolder = ExecutableWriterHolder<BYTE>(to, pDesc->m_pFuncEnd - pDesc->m_pFuncStart);
+                barrierWriterHolder = ExecutableWriterHolder<BYTE>(to, barrierSize);
                 to = barrierWriterHolder.GetRW();
             }
             GWB_PATCH_OFFSET(g_lowest_address);