seccomp: drop SECCOMP_MEMORY_DENY_WRITE_EXECUTE_BROKEN, add test for shmat

seccomp: drop SECCOMP_MEMORY_DENY_WRITE_EXECUTE_BROKEN, add test for shmat

SECCOMP_MEMORY_DENY_WRITE_EXECUTE_BROKEN was conflating two separate things:
1. whether shmat/shmdt/shmget can be filtered (if ipc multiplexer is used, they can not)
2. whether we know this for the current architecture

For i386, shmat is implemented as ipc, so seccomp filter is "broken" for shmat,
but not for mmap, and SECCOMP_MEMORY_DENY_WRITE_EXECUTE_BROKEN cannot be used
to cover both cases. The define was only used for tests — not in the implementation
in seccomp-util.c. So let's get rid of SECCOMP_MEMORY_DENY_WRITE_EXECUTE_BROKEN
and encode the right condition directly in tests.