Updated golang/x/net. Also updated golang.org/x/sys, as required by

hack/lint-dependencies.sh.

vendor/golang.org/x/sys/unix/gccgo.go

@@ -12,10 +12,8 @@ import "syscall"
 // We can't use the gc-syntax .s files for gccgo. On the plus side
 // much of the functionality can be written directly in Go.
 
-//extern gccgoRealSyscallNoError
 func realSyscallNoError(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r uintptr)
 
-//extern gccgoRealSyscall
 func realSyscall(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r, errno uintptr)
 
 func SyscallNoError(trap, a1, a2, a3 uintptr) (r1, r2 uintptr) {