syscallx: use golang.org/x/sys

[zhsj]

use Readlink from golang.org/x/sys/{unix,windows}

syscall_unix.go is also changed because golang.org/x/sys is
preferred than syscall package.

[estesp]

Given this will no longer use the syscall import, I think it might be better to delete these files/this directory and move the call to sys/unix.Readlink into the sysx directory (where it is being used from file_posix.go); I guess you would need to create 2 files there given there is windows support, but maybe a readlink_unix.go and a readlink_windows.go in that directory with the calls to sys/unix and sys/windows respectively. I think this directory was temporary while the syscall itself was implemented for Windows before going upstream.

[zhsj]

I read the code again, I find the duplicated Readlink on windows is intentional. So let's close this PR.
#113

[zhsj]

Although @darstahl said the fork of Readlink was temporary, but I didn't find upstream commits addressing any bug. So maybe this is still not upstream yet.