Use /proc/self/auxv for ARM feature detection when possible.

This is generally preferable because it works in QEMU chroots and
the information is significantly easier to process.

We still need to keep the /proc/cpuinfo code around, however, as
Android does not always allow access to /proc/self/auxv...

This patch also enhances the MONO_CPU_ARCH environment variable
to support the various new features we support.