Bring back weakly-linked fatalError to Mutex.cpp

Author: MaxDesiatov

stdlib/public/Concurrency/Mutex.cpp

@@ -18,3 +18,8 @@
 #ifdef SWIFT_STDLIB_SINGLE_THREADED_RUNTIME
   #include "swift/Runtime/MutexSingleThreaded.h"
 #endif
+
+__attribute__ ((weak))
+SWIFT_NORETURN void swift::fatalError(uint32_t flags, const char *format, ...) {
+  abort();
+}