Build ThreadX for more CPUs and enable armclang

Old mentioned upstream issues are now sorted, enable ThreadX builds for
more CPUs. Cortex-M52 will use the Cortex-M55 path.

Enable armclang v6 builds again.

Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>
Change-Id: I374492dee7b2a6ae02b21f84d0e8bb6434ab729b