cmd/link: pass provided ldflags when testing whether an ldflag is supported

It's possible for one of the ldflags to cause the compiler driver to
use a different linker than the default, so we need to make sure that
the flag is supported by whichever linker is specified.

Fixes #27110.

Change-Id: Ic0c51b886e34344d324e68cbf6673b168c14992f
Reviewed-on: https://go-review.googlesource.com/130316
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/link/internal/ld/lib.go

@@ -1366,7 +1366,12 @@ func linkerFlagSupported(linker, flag string) bool {
 		}
 	})
 
-	cmd := exec.Command(linker, flag, "trivial.c")
+	var flags []string
+	flags = append(flags, ldflag...)
+	flags = append(flags, strings.Fields(*flagExtldflags)...)
+	flags = append(flags, flag, "trivial.c")
+
+	cmd := exec.Command(linker, flags...)
 	cmd.Dir = *flagTmpdir
 	cmd.Env = append([]string{"LC_ALL=C"}, os.Environ()...)
 	out, err := cmd.CombinedOutput()