cmd/cgo: make _cgo_flags consistent across runs

    
    The go tool will pass -I objdir as one of the flags, where objdir is
    the temporary build directory. Remove that from _cgo_flags: we don't
    need it, and it will be different each time.
    
    Sort the flags to avoid the unpredictable map iteration order.
    
    This matters for gccgo because for a package that uses cgo, the go
    tool when building for gccgo will store the _cgo_flags file in the
    archive. That means that we want to generate identical _cgo_flags for
    every run.
    
    The test for this is the cmd/go testsuite, to follow in a future CL.
    
    Reviewed-on: https://go-review.googlesource.com/45692

From-SVN: r249199