runtime: fix build on freebsd/arm

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

How to generate defs-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=minux.ma, iant
R=iant, minux.ma, dave
CC=golang-codereviews
https://golang.org/cl/59580045

src/pkg/runtime/defs_freebsd_arm.h

@@ -159,10 +159,12 @@ struct Ucontext {
 struct Timespec {
 	int64	tv_sec;
 	int32	tv_nsec;
+	byte	Pad_cgo_0[4];
 };
 struct Timeval {
 	int64	tv_sec;
 	int32	tv_usec;
+	byte	Pad_cgo_0[4];
 };
 struct Itimerval {
 	Timeval	it_interval;