Ensure embedded FlatBuffers binary schemas are 8-byte aligned

In `src/idl_gen_cpp.cpp`, code generated by `flatc --bfbs-gen-embed` emits `static const uint8_t bfbsData[...] = { ... };`. Because `uint8_t` has 1-byte alignment, the array may be placed at an unaligned address in `.rodata`, triggering alignment traps under Clang UBSan on Arm when accessed via FlatBuffers root accessors.

Add `alignas(uint64_t)` to the generated `bfbsData` array declaration in `GenerateBinarySchema` so embedded binary schemas are 8-byte aligned across all architectures.

PiperOrigin-RevId: 1003962636

tests/monster_test_bfbs_generated.h

@@ -12,7 +12,7 @@ namespace Example {
 struct MonsterBinarySchema {
   static const uint8_t *data() {
     // Buffer containing the binary schema.
-    alignas(uint64_t) static const uint8_t bfbsData[14520] = {
+    static const uint8_t bfbsData[14520] = {
       0x20,0x00,0x00,0x00,0x42,0x46,0x42,0x53,0x00,0x00,0x00,0x00,0x14,0x00,0x20,0x00,0x04,0x00,0x08,0x00,
       0x0C,0x00,0x10,0x00,0x14,0x00,0x18,0x00,0x00,0x00,0x1C,0x00,0x14,0x00,0x00,0x00,0x58,0x00,0x00,0x00,
       0x34,0x00,0x00,0x00,0x24,0x00,0x00,0x00,0x18,0x00,0x00,0x00,0x70,0x0D,0x00,0x00,0x08,0x00,0x00,0x00,