Fix ARM bare metal driver to support atomics

The new bare metal support only supports the single thread model. This causes
the builtin atomic functions (e.g.: __atomic_fetch_add) to not generate
thread-safe assembly for these operations, which breaks our firmware. We target
bare metal, and need to atomically modify variables in our interrupt routines,
and task threads.

Internally, the -mthread-model flag determines whether to lower or expand
atomic operations (see D4984).

This change removes the overridden thread model methods, and instead relies on
the base ToolChain class to validate the thread model (which already includes
logic to validate single thread model support). If the single thread model is
required, the -mthread-model flag will have to be provided.

As a workaround "-mthread-model posix" could be provided, but it only works due
to a bug in the validation of the -mthread-model flag (separate patch coming to
fix this).

https://reviews.llvm.org/D37493

Patch by: Ian Tessier!

llvm-svn: 312651