runtime: fix "SysReserve returned unaligned address" bug on 32-bit systems
runtime: fix "SysReserve returned unaligned address" bug on 32-bit systems

R=rsc
CC=golang-dev
http://codereview.appspot.com/5642064

Committer: Nigel Tao <58ae08b3356928ff9c67562b4719487d54fe8348@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang