Reserve 12 more bytes in sem_t to give room for future implementation
improvements. This gets us to a 16 bytes sem_t. Glibc uses 32 bytes (16
actual use + 16 reserved), while OpenBSD has 16 bytes (out of which 4
are for padding).
Bug: 14587103
Bug: 12875898
Change-Id: Id835cc5abf874c651e6b5ad5b8f29c9d6ab08d5a