Use union to make alignement robust.

Suggested-by: vjpai@google.com

src/core/lib/gpr/mpscq.h

@@ -38,9 +38,11 @@ typedef struct gpr_mpscq_node {
 
 // Actual queue type
 typedef struct gpr_mpscq {
-  gpr_atm head;
   // make sure head & tail don't share a cacheline
-  char padding[GPR_CACHELINE_SIZE - sizeof(head)];
+  union {
+    char padding[GPR_CACHELINE_SIZE];
+    gpr_atm head;
+  };
   gpr_mpscq_node* tail;
   gpr_mpscq_node stub;
 } gpr_mpscq;

src/core/lib/iomgr/ev_epoll1_linux.cc

@@ -193,9 +193,13 @@ struct grpc_pollset_worker {
 #define MAX_NEIGHBORHOODS 1024
 
 typedef struct pollset_neighborhood {
-  gpr_mu mu;
-  grpc_pollset* active_root;
-  char pad[GPR_CACHELINE_SIZE - sizeof(mu) - sizeof(active_root)];
+  union {
+    char pad[GPR_CACHELINE_SIZE];
+    struct {
+      gpr_mu mu;
+      grpc_pollset* active_root;
+    };
+  };
 } pollset_neighborhood;
 
 struct grpc_pollset {