syscall: fix Fchdir on js/wasm

NodeJS does not support fchdir so it has to be emulated with chdir by
saving the path when opening a directory.

However, if the path opened is relative, saving this path is not
sufficient, because after changing the working directory the path
does not resolve correctly any more, thus a subsequent fd.Chdir() fails.

This change fixes the issue by resolving a relative path when
opening the directory and saving the absolute path instead.

Fixes #37448

Change-Id: Id6bc8c4232b0019fc11e850599a526336608ce54
Reviewed-on: https://go-review.googlesource.com/c/go/+/221717


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>