cmd/link: do not pass -no-pie to host linker when -linkshared is passed

As the comment above the code I'm changing says, when building with
-buildmode=exe, the default compiler flags produce code incompatible with PIE.
But when -linkshared is passed, the default compiler flags are not used so this
does not apply. And now I've found a system (linux/arm64 with glibc 2.27) where
this combination of flags causes a problem, albeit for reasons I don't really
understand, so stop passing -no-pie when -linkshared is passed.

Change-Id: I412ec7941dc0cb89e6d1b171fc29288aadcb9f20
Reviewed-on: https://go-review.googlesource.com/104815
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1266,7 +1266,7 @@ func (ctxt *Link) hostlink() {
 	// does not work, the resulting programs will not run. See
 	// issue #17847. To avoid this problem pass -no-pie to the
 	// toolchain if it is supported.
-	if ctxt.BuildMode == BuildModeExe {
+	if ctxt.BuildMode == BuildModeExe && !ctxt.linkShared {
 		src := filepath.Join(*flagTmpdir, "trivial.c")
 		if err := ioutil.WriteFile(src, []byte("int main() { return 0; }"), 0666); err != nil {
 			Errorf(nil, "WriteFile trivial.c failed: %v", err)