cmd/go: don't lose last flag from _cgo_flags

The quoting code that read _cgo_flags, currently only in the gccgo
    version of cmd/go, was losing the last flag read from the file.
    
    Fixes golang/go#23666
    
    Reviewed-on: https://go-review.googlesource.com/91655

From-SVN: r257373

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-981e6621bcd48670d0b58e51e9eeffe549725378
+9057b8f71e6078f140938fe60be9aaa7d59a3a2b
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/cmd/go/internal/work/gccgo.go

@@ -258,6 +258,9 @@ func (tools gccgoToolchain) link(b *Builder, root *Action, out, importcfg string
 					nl = append(nl, b)
 					start = false
 				}
+				if len(nl) > 0 {
+					cgoldflags = append(cgoldflags, string(nl))
+				}
 			}
 		}
 		return nil