[Build][Runtime] Replace SWIFT_STDLIB_SINGLE_THREADED_RUNTIME.

SWIFT_STDLIB_SINGLE_THREADED_RUNTIME is too much of a blunt instrument here.
It covers both the Concurrency runtime and the rest of the runtime, but we'd
like to be able to have e.g. a single-threaded Concurrency runtime while
the rest of the runtime is still thread safe (for instance).

So: rename it to SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY and make it just
control the Concurrency runtime, then add a SWIFT_STDLIB_THREADING_PACKAGE
setting at the CMake/build-script level, which defines
SWIFT_STDLIB_THREADING_xxx where xxx depends on the chosen threading package.

This is especially useful on systems where there may be a choice of threading
package that you could use.

rdar://90776105

stdlib/public/Concurrency/TaskGroup.cpp

@@ -34,6 +34,11 @@
 #include "queue" // TODO: remove and replace with usage of our mpsc queue
 #include <atomic>
 #include <new>
+
+#if !SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY
+#include <mutex>
+#endif
+
 #include <assert.h>
 #if SWIFT_CONCURRENCY_ENABLE_DISPATCH
 #include <dispatch/dispatch.h>
@@ -280,7 +285,7 @@ public:
 
 private:
 
-#if !SWIFT_STDLIB_SINGLE_THREADED_RUNTIME
+#if !SWIFT_STDLIB_SINGLE_THREADED_CONCURRENCY
   // TODO: move to lockless via the status atomic (make readyQueue an mpsc_queue_t<ReadyQueueItem>)
   mutable std::mutex mutex_;