Use aarch64-linux-android-4.9 for arm64 build (attempt #2)

GCC: 4.9 (which supports -fstack-protector)
Binutils: 2.24 (which supports gc-sections)
GDB: 7.7

NDK libraries are still picked up from prebuilts/ndk/*/4.8/*
GCC has been patched to disable codegen for calling
 __cxa_throw_bad_array_new_length.

Change-Id: Ie647fc4c6b227d6bee792f04d5c2f02eb0099559