Fix GCC build with PROTOBUF_USE_DLLS enabled

GCC does not allow mixing __attribute__(()) syntax alignas()
syntax. Re-use approach from chromium base/compiler_specific.h

src/google/protobuf/arena.h

@@ -248,7 +248,7 @@ struct ArenaOptions {
 // well as protobuf container types like RepeatedPtrField and Map. The protocol
 // is internal to protobuf and is not guaranteed to be stable. Non-proto types
 // should not rely on this protocol.
-class PROTOBUF_EXPORT alignas(8) Arena final {
+class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8) Arena final {
  public:
   // Arena constructor taking custom options. See ArenaOptions below for
   // descriptions of the options available.

src/google/protobuf/port_def.inc

@@ -420,3 +420,32 @@
 #else
 #define PROTOBUF_ASSUME(pred) GOOGLE_DCHECK(pred)
 #endif
+
+// Specify memory alignment for structs, classes, etc.
+// Use like:
+//   class PROTOBUF_ALIGNAS(16) MyClass { ... }
+//   PROTOBUF_ALIGNAS(16) int array[4];
+//
+// In most places you can use the C++11 keyword "alignas", which is preferred.
+//
+// But compilers have trouble mixing __attribute__((...)) syntax with
+// alignas(...) syntax.
+//
+// Doesn't work in clang or gcc:
+//   struct alignas(16) __attribute__((packed)) S { char c; };
+// Works in clang but not gcc:
+//   struct __attribute__((packed)) alignas(16) S2 { char c; };
+// Works in clang and gcc:
+//   struct alignas(16) S3 { char c; } __attribute__((packed));
+//
+// There are also some attributes that must be specified *before* a class
+// definition: visibility (used for exporting functions/classes) is one of
+// these attributes. This means that it is not possible to use alignas() with a
+// class that is marked as exported.
+#if defined(_MSC_VER)
+#define PROTOBUF_ALIGNAS(byte_alignment) __declspec(align(byte_alignment))
+#elif defined(__GNUC__)
+#define PROTOBUF_ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment)))
+#else
+#define PROTOBUF_ALIGNAS(byte_alignment) alignas(byte_alignment)
+#endif

src/google/protobuf/port_undef.inc

@@ -68,6 +68,7 @@
 #undef PROTOBUF_ASSUME
 #undef PROTOBUF_EXPORT_TEMPLATE_DECLARE
 #undef PROTOBUF_EXPORT_TEMPLATE_DEFINE
+#undef PROTOBUF_ALIGNAS
 
 // Restore macro that may have been #undef'd in port_def.inc.
 #ifdef _MSC_VER