[nacl] force nacl to use libgcc for unwinding

While testing llvm-libunwind for unwinding, we ran into a build failure caused
by nacl being built with --rtlib=libgcc, and requiring libgcc for unwinding.
Force it with --unwindlib=libgcc.

Bug:870480

Change-Id: I4a1b601cd254b6a77c2612342e783cc2e0b083fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1836152Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Derek Schuff <dschuff@chromium.org>
Auto-Submit: Jian Cai <jiancai@google.com>
Commit-Queue: Jian Cai <jiancai@google.com>
Cr-Commit-Position: refs/heads/master@{#708074}

components/nacl/loader/BUILD.gn

@@ -139,7 +139,10 @@ if (is_linux) {
     if (is_chromeos) {
       # NaCl is not working with compiler-rt in ChromeOS.
       # Force libgcc as a workaround. See https://crbug.com/761103
-      ldflags = [ "-rtlib=libgcc" ]
+      ldflags = [
+        "-rtlib=libgcc",
+        "--unwindlib=libgcc",
+      ]
 
       # nacl_helper needs to link with libgcc_s. We already have -lgcc_s in
       # the final link command line, but it was placed within "--as-needed"