syscall: fix build on freebsd/arm

This CL is in preparation to make cgo work on freebsd/arm.

It's just for fixing build fails on freebsd/arm, we still need to
update z-files later for fixing several package test fails.

How to generate z-files on freebsd/arm in the bootstrapping phase:
1. run freebsd on appropriate arm-eabi platforms
2. both syscall z-files and runtime def-files in the current tree are
   broken about EABI padding, fix them by hand
3. run make.bash again to build $GOTOOLDIR/cgo
4. use $GOTOOLDIR/cgo directly

LGTM=iant
R=iant, dave
CC=golang-codereviews
https://golang.org/cl/59490052

src/pkg/syscall/ztypes_freebsd_arm.go

@@ -19,13 +19,15 @@ type (
 )
 
 type Timespec struct {
-	Sec  int64
-	Nsec int32
+	Sec       int64
+	Nsec      int32
+	Pad_cgo_0 [4]byte
 }
 
 type Timeval struct {
-	Sec  int64
-	Usec int32
+	Sec       int64
+	Usec      int32
+	Pad_cgo_0 [4]byte
 }
 
 type Rusage struct {