Add support for the Linux/m68k platform

Add necessary support code to recognize m68k as an architecture;
use GCC builtin atomics like avr32 does. Copy avr32 header, since
it was hinted that including it from m68k was not liked.

This is not needed in Qt5 because it’s said to automatically use
the GCC atomic builtins.

Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=660963

Task-number: QTBUG-28237
Change-Id: I6c51405c47549c904899a6971b6cd34b8239c642
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>