Bump microblaze minimum kernel requirement to 3.15?

[Florian Weimer]

Earlier kernels do not provide preadv and pwritev, and our emulation is 
severely broken; see bug 22442.

I'd like to simply bump the kernel version requirement for microblaze 
instead of trying to fix the fallback code (probably using mmap, but it 
will still not be cancel-safe).

Another alternative would be to rip out the emulation code completely 
and change the system call wrappers to return ENOSYS if the system call 
is not defined.

Thanks,
Florian