Re: <sys/membarrier.h>

> yeah, "all the choices suck". varargs has caused android (and android
> developers) pain in the past too, so i'm tempted to go for three
> arguments, but then callers are going to need an #if to support musl,
> at which point everyone may as well have stuck with syscall() :-(

We can provide C++ overloads for two and three arguments.  With GCC, we
can use __builtin_va_arg_pack and __builtin_va_arg_pack_len to emulate
that in C.  It's not supported by Clang.

Thanks,
Florian