Pick gnu-libstdc++ based on TARGET_GCC_VERSION

Previously we have only one set of include/lib paths for
LOCAL_NDK_STL_VARIANT:=gnustl_static regardless of GCC
version, which is wrong because each GCC version
come with its own libstdc++.

Change-Id: I2a01c2120b6948aedce00e2f8d08dfc6932126dd