fix arm64 alignment issues in signal()

Author: ryanschneider

src/cstd.zig

@@ -538,7 +538,12 @@ const SignalFn = switch (builtin.zig_backend) {
     .stage1 => fn(c_int) callconv(.C) void,
     else => *const fn(c_int) callconv(.C) void,
 };
-export fn signal(sig: c_int, func: SignalFn) callconv(.C) ?SignalFn {
+const SignalResult = packed union {
+    handler: ?SignalFn,
+    err: isize,
+};
+export fn signal(sig: c_int, func: SignalFn) callconv(.C) SignalResult {
+    std.debug.assert(@sizeOf(SignalResult) == @sizeOf(?SignalFn));
     if (builtin.os.tag == .linux) {
         var action = std.os.Sigaction{
             .handler = .{ .handler = func },
@@ -552,12 +557,10 @@ export fn signal(sig: c_int, func: SignalFn) callconv(.C) ?SignalFn {
             &action,
             &old_action,
         ))) {
-            .SUCCESS => return old_action.handler.handler,
+            .SUCCESS => return SignalResult{ .handler = old_action.handler.handler },
             else => |e| {
                 errno = @enumToInt(e);
-                // translate-c having a hard time with this one
-                //return c.SIG_ERR;
-                return @intToPtr(?SignalFn, @bitCast(usize, @as(isize, -1)));
+                return SignalResult{ .err = -1 };
             },
         }
     }