Twincpu: Adjust App CMakeLists.txt to support multi-core build

Disable building single Armv8-M specific files in multi-core
topology.
Disable veneer setting in multi-core topology.

Change-Id: Ibc5d0a6cd4d6c8136cb0a99f124ef756d3f8cc5b
Signed-off-by: David Hu <david.hu@arm.com>