PR go/68255
    cmd/go: always use --whole-archive for gccgo packages
    
    This is a backport of https://golang.org/cl/16775.
    
    This is, in effect, what the gc toolchain does.  It fixes cases where Go
    code refers to a C global variable; without this, if the global variable
    was the only thing visible in the C code, the generated cgo file might
    not get pulled in from the archive, leaving the Go variable
    uninitialized.
    
    This was reported against gccgo as https://gcc.gnu.org/PR68255 .
    
    Reviewed-on: https://go-review.googlesource.com/16778

From-SVN: r230120