MFC r335201 (by emaste@):

linuxulator: do not include legacy syscalls on arm64

Existing linuxulator platforms (i386, amd64) support legacy syscalls,
such as non-*at ones like open, but arm64 and other new platforms do
not.

Wrap these in #ifdef LINUX_LEGACY_SYSCALLS, #defined in the MD linux.h
files. We may need finer grained control in the future but this is
sufficient for now.