change unreachable{} to @unreachable()

instead of a container init expression, it's a builtin
function call.

std/linux.zig

@@ -299,7 +299,7 @@ pub fn lseek(fd: i32, offset: usize, ref_pos: usize) -> usize {
 
 pub fn exit(status: i32) -> unreachable {
     arch.syscall1(arch.SYS_exit, usize(status));
-    unreachable{}
+    @unreachable()
 }
 
 pub fn getrandom(buf: &u8, count: usize, flags: u32) -> usize {