cmd/go: fixes to gccgo support
cmd/go: fixes to gccgo support

* Use -fgo-pkgpath and -gccgopkgpath rather than -fgo-prefix
  and -gccgoprefix.
* Define GOPKGPATH when compiling .c or .s files for gccgo.
* Use -fgo-relative-import-path.
* Produce .o files for gccgo, not .[568] files.
* Pass -E when linking if using cgo.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6820064

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