[Linux] Disable fatalError() backtraces when the runtime backtracer is active.

There's no need for fatalError() to try to generate its own backtraces
when the runtime's backtracer is enabled.  Not only is the code it uses
more fragile but it also doesn't support async or inline frames and it
can't look-up symbols properly either.

rdar://117470489

test/Backtracing/FatalError.swift

@@ -37,6 +37,8 @@ struct FatalError {
   }
 }
 
+// CHECK-NOT: Current stack trace:
+
 // CHECK: *** Program crashed: {{Illegal instruction|System trap}} at 0x{{[0-9a-f]+}} ***
 
 // CHECK: Thread 0 {{(".*" )?}}crashed: