Pre-M and GCC compatibility for crtbegin.

We're going to start using the bionic sources for the NDK CRT
objects, so we need to avoid using symbols that weren't around in
early versions of Android. The NDK is currently building the CRT
objects with GCC as well (there were some segfaults that have yet to
be diagnosed), so move `__used` to the GCC compatible location.

Test: treehugger
Bug: None
Change-Id: I1f5c23eafadc2e3bc0b84bc3305f79a04d35c7d8