arm/gn: don't statically link libstdc++

As we're building whole systems and not binaries that are expected to
be copied around, we can dynamically link to libstdc++.

Change-Id: I1e54ca519b069388226f40c489c9194b870a6d3c
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-devtools/gn/gn_git.bb

@@ -31,6 +31,7 @@ do_configure() {
     python3 ${S}/build/gen.py \
         --platform=${@gn_platform("TARGET_OS", d)} \
         --out-path=${B} \
+        --no-static-libstdc++ \
         --no-strip
 }