Fix building gn on rosa2016.1

Commit d9f0d84840 ("Build with llvm11") removed "-stdlib=libc++".
Building of the gn utility which is used to build Chromium (and is not packaged) requires a newer libstdc++ than the one in rosa2016.1.
Fix building it by using libstdc++-gcc10. This is a better solution than the one removed by that commit because compiler flags will not influence the build of Chromium itlsef.

chromium-browser-stable.spec

@@ -154,6 +154,8 @@ BuildRequires:	libcxx-devel
 BuildRequires:	libcxxabi-devel
 BuildRequires:	lld
 %endif
+# for gn, not chromium itself
+BuildRequires:	stdc++-gcc10-devel
 # gn needs these
 BuildRequires:	flex
 %if %{with bundled_llvm}
@@ -576,6 +578,16 @@ CXXFLAGS="$CXXFLAGS -Wno-error=attributes -Wno-error=comment -Wno-error=unused-v
 %endif
 
 export PYTHON=%{__python}
+# libstdc++ in rosa2016.1 is too old to build gn, there are 2 ways to solve this:
+# 1) run bootstrap.py with --use-custom-libcxx, it will build and use bundled libc++
+# 2) use libstdc++-gcc10 (https://forum.rosalinux.ru/viewtopic.php?t=10047&p=104560)
+# Let's choose option 2.
+LDFLAGS="-lstdc++-gcc10" \
+CXXFLAGS="-nostdinc++ \
+          -Wno-unused-command-line-argument \
+          -I%{_includedir}/libstdc++-gcc10 \
+          -I%{_includedir}/libstdc++-gcc10/%{_host} \
+         " \
 %{__python} tools/gn/bootstrap/bootstrap.py --gn-gen-args='%{gn_config}'
 out/Release/gn gen --script-executable=%{__python} --args='%{gn_config}' out/Release