Direct syscalls using syscall.Syscall(SYS_*, ...) should no longer be
used on darwin, see [1]. Instead, use the FcntlFstore libSystem wrapper
provided by the golang.org/x/sys/unix package to implement
preallocFixed.
[1] https://golang.org/doc/go1.12#darwin
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>