disable aarch64 backend in release tarballs

[190n]

Zig Version

0.14.0

Steps to Reproduce and Observed Output

Try compiling a simple program with the aarch64 backend. The compiler crashes:

const std = @import("std");

pub fn main() void {
    std.debug.print("hello world", .{});
}

$ zig build-exe -target aarch64-linux-gnu -fno-llvm hello.zig
zsh: bus error  zig build-exe -target aarch64-linux-gnu -fno-llvm hello.zig

Expected Output

error: compiler backend unavailable for the specified target

I understand this backend isn't expected to be usable at the moment; thus there should be a clear error indicating as such rather than letting it crash.

[alexrp]

We are likely going to just delete the unfinished and unmaintained backends (arm, aarch64, sparc64) as they will require a full rewrite anyway.