build: add GO_ prefix to LDFLAGS and GCFLAGS
build: add GO_ prefix to LDFLAGS and GCFLAGS

Build environments will often define stock LDFLAGS
that are not compatible with the gc ld, causing
non-obvious failures midway through the build.

R=golang-dev, rsc, minux.ma, rsc
CC=golang-dev
http://codereview.appspot.com/5724044

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