Turns on -ldstex2cas by default for 32-bit ARM. However, adds bailing on conversion for predication or for pair operations, which are not yet handled.
Fixes -no_ldstex2cas to properly mangle stolen register usage in exclusive operations.
Adds proc_get_architecture() for determining whether the processor supports load-acquire operations.
Tested on the client.ldstex test on ARM Linux.
Issue: #1698