Add src/corelib/arch/qatomic_unix.h and qatomic_unix.cpp

Add src/corelib/arch/qatomic_unix.h and qatomic_unix.cpp

This provides a fallback implementation on UNIX when the Q_PROCESSOR_*
and Q_CC_*/Q_COMPILER_* checks fail to find an implementation.

Note that we always compile qatomic_unix.cpp, but code is only included
when QATOMIC_UNIX_H is defined (meaning the checks above did not find an
implementation).

Change-Id: I8ce047847206003b4fa96eb3fb76b1c2ffbc2dfc
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>