cmd/go: add -flat_namespace to LDFLAGS whitelist

Fixes #26173

Change-Id: I032551f63b359c8cbb7296931e1957d2bff8f328
Reviewed-on: https://go-review.googlesource.com/121819
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/cmd/go/internal/work/security.go

@@ -136,6 +136,7 @@ var validLinkerFlags = []*regexp.Regexp{
 	re(`-f(no-)?(pic|PIC|pie|PIE)`),
 	re(`-f(no-)?openmp(-simd)?`),
 	re(`-fsanitize=([^@\-].*)`),
+	re(`-flat_namespace`),
 	re(`-g([^@\-].*)?`),
 	re(`-headerpad_max_install_names`),
 	re(`-m(abi|arch|cpu|fpu|tune)=([^@\-].*)`),