syscall: implement Getwd using getcwd from libSystem on darwin

Directly wrap the getcwd implementation provided by libSystem.dylib on
darwin and use it to implement Getwd like on the BSDs. This allows to
drop the custom implementation using getAttrList and to merge the
implementation of Getwd for darwin and the BSDs in syscall_bsd.go.

Same as CL 257497 did for golang.org/x/sys/unix

Change-Id: If30390c4c17cd463bb8fdcb5465f40d6fa11f391
Reviewed-on: https://go-review.googlesource.com/c/go/+/257637


Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>