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

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