cmd/internal/ld: change Cpos to not flush the output buffer

DWARF generation appears to assume Cpos is cheap and this makes linking godoc
about 8% faster and linking the standard library into a single shared library
about 22% faster on my machine.

Updates golang#10571

Change-Id: I3f81efd0174e356716e7971c4f59810b72378177

src/cmd/internal/ld/lib.go

@@ -1573,12 +1573,11 @@ func Cflush() {
 }
 
 func Cpos() int64 {
-	Cflush()
 	off, err := coutbuf.f.Seek(0, 1)
 	if err != nil {
 		Exitf("seeking in output [0, 1]: %v", err)
 	}
-	return off
+	return off + int64(coutbuf.Buffered())
 }
 
 func Cseek(p int64) {