unix: remove syscall constants on Solaris
unix: remove syscall constants on Solaris

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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,golang/sys