POSIX link() syscall only takes two arguments (no flags)

The signature is documented as:

   int link(const char *, const char *);

(see https://man7.org/linux/man-pages/man2/link.2.html or https://man.netbsd.org/link.2)

And its not some Linux extension, the [syscall
implementation](21b136cc63/fs/namei.c (L4794-L4797))
only expects two arguments too.

It probably *should* have a flags parameter, but its too late now.

I am a bit surprised that linking glibc or musl against code that invokes
a 'link' with three parameters doesn't fail (at least, I couldn't get any
local test cases to trigger a compile or link error).

The test case in std/posix/test.zig is currently disabled, but if I
manually enable it, it works with this change.

lib/std/os/linux.zig

@@ -1339,13 +1339,12 @@ pub fn tgkill(tgid: pid_t, tid: pid_t, sig: i32) usize {
     return syscall3(.tgkill, @as(usize, @bitCast(@as(isize, tgid))), @as(usize, @bitCast(@as(isize, tid))), @as(usize, @bitCast(@as(isize, sig))));
 }
 
-pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) usize {
+pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8) usize {
     if (@hasField(SYS, "link")) {
-        return syscall3(
+        return syscall2(
             .link,
             @intFromPtr(oldpath),
             @intFromPtr(newpath),
-            @as(usize, @bitCast(@as(isize, flags))),
         );
     } else {
         return syscall5(
@@ -1354,7 +1353,7 @@ pub fn link(oldpath: [*:0]const u8, newpath: [*:0]const u8, flags: i32) usize {
             @intFromPtr(oldpath),
             @as(usize, @bitCast(@as(isize, AT.FDCWD))),
             @intFromPtr(newpath),
-            @as(usize, @bitCast(@as(isize, flags))),
+            0,
         );
     }
 }