unix: add utimensat libc wrapper on darwin

Direct syscalls are no longer supported on darwin (CL 250437), so
utimensat can be implemented as a wrapper around the libc function.
The utimensat function was added in macOS 10.13 and Go 1.17 dropped
support for macOS 10.12.

This also allows to drop the fallback to setattrlistTimes which was
used to set timestamps with nanosecond resolution before utimensat could
be used, see golang/go#22528 and CL 74952.

Change-Id: I206291277e6f7200ca7a659e29075968647779a6
Reviewed-on: https://go-review.googlesource.com/c/sys/+/251737
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/darwin_arm64_test.go

@@ -118,7 +118,6 @@ var darwinTests = [...]darwinTest{
 	{"sendfile", libc_sendfile_trampoline_addr},
 	{"sendmsg", libc_sendmsg_trampoline_addr},
 	{"sendto", libc_sendto_trampoline_addr},
-	{"setattrlist", libc_setattrlist_trampoline_addr},
 	{"setegid", libc_setegid_trampoline_addr},
 	{"seteuid", libc_seteuid_trampoline_addr},
 	{"setgid", libc_setgid_trampoline_addr},
@@ -154,6 +153,7 @@ var darwinTests = [...]darwinTest{
 	{"unlink", libc_unlink_trampoline_addr},
 	{"unlinkat", libc_unlinkat_trampoline_addr},
 	{"unmount", libc_unmount_trampoline_addr},
+	{"utimensat", libc_utimensat_trampoline_addr},
 	{"utimes", libc_utimes_trampoline_addr},
 	{"wait4", libc_wait4_trampoline_addr},
 	{"write", libc_write_trampoline_addr},