PR go/66904
    cmd/go: fix "#cgo pkg-config:" comments with gccgo
    
    Copy of https://golang.org/cl/18790 by Michael Hudson-Doyle.
    
    The unique difficulty of #cgo pkg-config is that the linker flags are recorded
    when the package is compiled but (obviously) must be used when the package is
    linked into an executable -- so the flags need to be stored on disk somewhere.
    As it happens cgo already writes out a _cgo_flags file: nothing uses it
    currently, but this change adds it to the lib$pkg.a file when compiling a
    package, reads it out when linking (and passes a version of the .a file with
    _cgo_flags stripped out of it to the linker). It's all fairly ugly but it works
    and I can't really think of any way of reducing the essential level of
    ugliness.
    
    Update golang/go#11739.
    GCC PR 66904.

    Reviewed-on: https://go-review.googlesource.com/19431

From-SVN: r233290