libphobos: core.atomic should have fallback when there's no libatomic.

libphobos: core.atomic should have fallback when there's no libatomic.

libphobos/ChangeLog:

2019-04-20  Iain Buclaw  <ibuclaw@gdcproject.org>

	PR d/89293
	* libdruntime/core/atomic.d (casImpl): Remove static assert for
	GNU_Have_Atomics, add static path to handle missing atomic support.
	(atomicLoad): Likewise.
	(atomicStore): Likewise.
	(atomicFence):  Likewise.
	(atomicMutexHandle, AtomicMutex): Declare types.
	(_getAtomicMutex): New function.
	(getAtomicMutex): Declare.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@270470 138bc75d-0d04-0410-961f-82ee72b054a4