SYS_EXECVE and SYS_FCNTL were needed for compatibility reasons in
syscall, but they are not needed in x/sys/unix.

Change-Id: Ic355cf8785c83be51bc91ba84401bc969f8ae6b5
Reviewed-on: https://go-review.googlesource.com/84995
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>