Linux has dropped __NR_open from AArch64

[zachriggle]

While working on #883, one of the failing tests is cat for aarch64, because __NR_open is not defined in MUSL's headers for aarch64.

At first this seemed like a bug, but our current syscalls for it are e.g.:

$ constgrep -c aarch64 __NR_open
#define __NR_openat            56
#define __NR_open_by_handle_at 265
#define __NR_open              1024

The 1024 seems suspect. In fact, QEMU does not support this syscall anymore as of qemu/qemu@13756fb0, which landed in v2.6.0. This makes the unsupported syscall 1024 that has been popping up make more sense.

I think the best way to handle it is to have aarch64/linux/open.asm which uses the SYS_openat syscall.

[zachriggle]

Looks like these syscalls never existed.

https://groups.google.com/a/chromium.org/forum/#!msg/chromium-reviews/H3HL8e3KTjw/7KWVBSBOAwAJ