api: add Fchflags exception to more platforms

Unbreaks the build.

R=golang-codereviews, dsymonds
CC=golang-codereviews
https://golang.org/cl/53820045

api/except.txt

@@ -5,6 +5,10 @@ pkg syscall (darwin-amd64), func Fchflags(string, int) error
 pkg syscall (darwin-amd64-cgo), func Fchflags(string, int) error
 pkg syscall (freebsd-386), func Fchflags(string, int) error
 pkg syscall (freebsd-amd64), func Fchflags(string, int) error
+pkg syscall (freebsd-arm), func Fchflags(string, int) error
+pkg syscall (freebsd-arm-cgo), func Fchflags(string, int) error
+pkg syscall (netbsd-arm), func Fchflags(string, int) error
+pkg syscall (netbsd-arm-cgo), func Fchflags(string, int) error
 pkg testing, func RegisterCover(Cover)
 pkg text/template/parse, type DotNode bool
 pkg text/template/parse, type Node interface { Copy, String, Type }