Start adding support to build bits of our code using the Thumb-2
instruction set. Thumb-2 requires an if-then instruction to implement
conditional codes.

When building for ARM mode the it-then instructions do not generate any
assembled instruction as per the ARMv7-A Architecture Reference Manual, and
are safe to use.

While this allows the atomic instructions to be built, it doesn't mean we
fully support Thumb code. It works in small tests, but is still known to
fail in a large number of places.

While here add a check for the armv6t2 architecture.