Missing readlinkat()

[nyh]

We are missing an implementation of the readlinkat() function, and the __NR_readlinkat (267) syscall which uses it.
Golang applications appear to use this syscall during initialization, though do not appear to be hurt by the missing implementation:

$ scripts/build image=golang-example
$ scripts/run.py
OSv v0.24-517-g43800836
eth0: 192.168.122.15
syscall(): unimplemented system call 267
Hello, 世界
Go version: go1.9.5

Implementing this function should be pretty easy, modeled after the rest of the *at() functions in fs/vfs/main.cc.