pcem: fit graphics sources into C++ builds

pcem: fit graphics sources into C++ builds

WinUAE builds the imported PCem graphics sources as C++ files.
The MGA source includes C atomics directly, and the BT482 RAMDAC
source uses C allocation declarations through C++ headers.

Use C++ atomics when compiling MGA as C++, and include the C++
cstdlib wrapper where the BT482 RAMDAC needs those declarations.