cmd/go: fix handling of build tags for standard commands Otherwise if I add '+build !race' to e.g. src/cmd/fix/reflect_test.go, it does not work.
cmd/go: fix handling of build tags for standard commands
Otherwise if I add '+build !race' to e.g. src/cmd/fix/reflect_test.go,
it does not work.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6642044

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