We only need it for MAX_ERRNO, and it's time we had somewhere to put
the little assembler utility macros we've been putting off writing.
Change-Id: I9354d2e0dc47c689296a34b5b229fc9ba75f1a83
(aarch64 kernels only have the newer system calls.)
Also expose the new functionality that's exposed by glibc in our header files.
Change-Id: I45d2d168a03f88723d1f7fbf634701006a4843c5