Use $CFG_ARCH even if src/corelib/arch/$CFG_ARCH does not exist

Use $CFG_ARCH even if src/corelib/arch/$CFG_ARCH does not exist

Recent commits in src/corelib/arch have removed empty directories,
which cause configure to assume that the architecture is not supported.
This assumption is wrong, due to changes in
src/corelib/thread/qbasicatomic.h to include the correct QAtomic*
implementation based on compiler pre-defined macros instead of the
architecture found by configure.

Change-Id: I026958fb9af8af62e295e68a4643c37ac9395dc6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>