GN: link executables with -rdynamic on Linux.

-rdynamic makes sure the executable exposes all its symbols, which allows things like DM's built-in stack trace dumper to show symbols instead of just addresses.  We do this in GYP too.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2339983005

Review-Url: https://codereview.chromium.org/2339983005

gn/BUILD.gn

@@ -153,7 +153,10 @@ config("executable") {
   if (is_mac) {
     ldflags = [ "-Wl,-rpath,@loader_path/." ]
   } else if (is_linux) {
-    ldflags = [ "-Wl,-rpath,\$ORIGIN" ]
+    ldflags = [
+      "-rdynamic",
+      "-Wl,-rpath,\$ORIGIN",
+    ]
   }
 }