runtime: update godefs usage comment for GOARCH=386 GOOS={darwin,freebsd}

When trying to regenerate src/pkg/runtime/darwin/386/defs.h
on a 64 bit capable Snow Leopard (OS X 10.6.3) system I
needed to add -f -m32 to godefs, as this OS and hardware
combination defaults to 64 bit compilation.

For safety, make the same change to the 32 bit FreeBSD
instructions in .../freebsd/defs.c.  (Tested OK and no
problems introduced.)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/1052042

src/pkg/runtime/darwin/386/defs.h

-// godefs defs.c
+// godefs -f -m32 defs.c
 
 // MACHINE GENERATED - DO NOT EDIT.
 

src/pkg/runtime/darwin/defs.c

@@ -6,7 +6,7 @@
  * Input to godefs.
  *
 	godefs -f -m64 defs.c >amd64/defs.h
-	godefs defs.c >386/defs.h
+	godefs -f -m32 defs.c >386/defs.h
  */
 
 #define __DARWIN_UNIX03 0

src/pkg/runtime/freebsd/386/defs.h

-// godefs defs.c
+// godefs -f -m32 defs.c
 
 // MACHINE GENERATED - DO NOT EDIT.
 

src/pkg/runtime/freebsd/defs.c

@@ -6,7 +6,7 @@
  * Input to godefs.
  *
 	godefs -f -m64 defs.c >amd64/defs.h
-	godefs defs.c >386/defs.h
+	godefs -f -m32 defs.c >386/defs.h
  */
 
 #include <sys/types.h>