RFC: Fixes required to build for Android 5 on AArch64

I don't expect these to be in the right shape for merging (except possibly the first one, which I hope is uncontroversial), but since I'm about to be minimally available for over a month I thought I'd at least get the patches out there.

The first patch replaces `index` with `strchr`. The bionic libc defines `index` as part of the [`ndk_cruft.cpp`](https://github.com/android/platform_bionic/blob/d0944d9ed38be0d74c3d8876f0f36b9ddc0ce38c/libc/bionic/ndk_cruft.cpp#L301) file, whose definitions are disabled on 64-bit platforms.

The second patch updates the `lxcmntent` compatability implementations to remove most of them. They are defined by the latest version of bionic (API 21). These would be more properly gated by a feature test--I'm just not sure if there is one. They might also be able to be gated by a particular bionic version, but I haven't looked into how that works, either.

The third patch bluntly hacks in the `-pie -fPIE` flags while compiling the lxc executable files. Starting with Android 5, you can't run non-posiiton-independent binaries.

Even with all this, LXC doesn't quite work--I can't actually mount multiple cgroups to a single mount point, so that part of `prepare.sh` fails.

(cf https://github.com/flar2/ElementalX-N9/pull/1)