runtime: fix mkversion to output valid path separators
runtime: fix mkversion to output valid path separators

In a GOROOT path a backslash is a path separator
not an escape character. For example, `C:\go`.
Fixes gotest error:
version.go:3: unknown escape sequence: g

R=rsc
CC=golang-dev
http://codereview.appspot.com/4437076

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

webfd/go-zh,rdp/rogerpack2005-golang,npe9/noahevans-go9,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,Triskite/willstone-goclone,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,bryanxu/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,webfd/go-zh,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,npe9/noahevans-go9,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,scirelli/scirelli-go,scirelli/scirelli-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rflanagan/reginaldflanagan-project1,scirelli/scirelli-go,bryanxu/go-zh,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,rflanagan/reginaldflanagan-project1,d0f/go-zh,scirelli/scirelli-go,npe9/noahevans-go9,npe9/noahevans-go9,scirelli/scirelli-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,Triskite/willstone-goclone,mhennings/marcohennings-go,scirelli/scirelli-go,d0f/go-zh,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,Triskite/willstone-goclone,rdp/rogerpack2005-golang,bryanxu/go-zh,npe9/noahevans-go9,bryanxu/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,bryanxu/go-zh,webfd/go-zh,npe9/noahevans-go9,npe9/noahevans-go9,sanjosh/sanjos100-tipc