Handle arbitrarily long symlink target in expandSymlinks() (#23650)

For now, `expandSymlinks()` can handle only symlinks with lengths up to
1024.

We can improve this logic and retry inside a loop with increasing
lengths until we succeed.

The same approach is used in
[Go](377646589d/src/os/file_unix.go (L446)),
[Rust](785eb65377/library/std/src/sys/pal/unix/fs.rs (L1700))
and [Nim's
`getCurrentDir()`](https://github.com/nim-lang/Nim/blob/devel/lib/std/private/ospaths2.nim#L877),
so maybe it's a good idea to use the same logic in `expandSymlinks()`
also.

(cherry picked from commit 3bda5fc840)

lib/std/private/ossymlinks.nim

@@ -66,11 +66,13 @@ proc expandSymlink*(symlinkPath: string): string {.noWeirdTarget.} =
   when defined(windows) or defined(nintendoswitch):
     result = symlinkPath
   else:
-    result = newString(maxSymlinkLen)
-    var len = readlink(symlinkPath, result.cstring, maxSymlinkLen)
-    if len < 0:
-      raiseOSError(osLastError(), symlinkPath)
-    if len > maxSymlinkLen:
-      result = newString(len+1)
-      len = readlink(symlinkPath, result.cstring, len)
-    setLen(result, len)
+    var bufLen = 1024
+    while true:
+      result = newString(bufLen)
+      let len = readlink(symlinkPath.cstring, result.cstring, bufLen)
+      if len < 0:
+        raiseOSError(osLastError(), symlinkPath)
+      if len < bufLen:
+        result.setLen(len)
+        break
+      bufLen = bufLen shl 1