Fix Linux non-x86/x86_64 compilation failure

[n8sh]

#49
libmir/mir#390

[9il]

Linux kernel headers define __NR_getrandom (and other syscall numbers). I have created an empty https://github.com/libmir/mir-linux-kernel for linux headers.

https://github.com/torvalds/linux can be used as reference.
We need an arch/<arch-name->/include/uapi/asm/unistd.h files and few small others that are imported by some archs.

D file name structure can looks like mir/linux/arch/<arch-name->/uapi/asm/unistd.d plus unified file mir/linux/asm/unistd.d that predifined arch version from predefined-versions . D does not support all architectures, but many of them are supported by LDC and D specification, see predefined-versions for details.

[9il]

If you do not have a time we can merge this for now.

[9il]

btw, ping @wilzbach

[n8sh]

If you do not have a time we can merge this for now.

I think adding the syscall numbers for the other architectures is the right thing to do, but I think for now we should unbreak the build.