syscall: exec_linux.go: support platforms without SYS_DUP2

Updates #9974

This change is in preparation for merging the arm64 platform.

Arm64 does not support SYS_DUP2 at all, so define a new constant to be
the minimum dup(2) version supported. This constant defaults to SYS_DUP2
on all existing platforms.

Change-Id: If405878105082c7c880f8541c1491970124c9ce4
Reviewed-on: https://go-review.googlesource.com/7123
Reviewed-by: Minux Ma <minux@golang.org>
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/syscall/syscall_linux_386.go

@@ -9,6 +9,8 @@ package syscall
 
 import "unsafe"
 
+const _SYS_dup = SYS_DUP2
+
 func Getpagesize() int { return 4096 }
 
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }