Use a trampoline if musttail not available

For the previous approach to work, we would have had to lock in the
generated code for every function that needs to make tail calls in
assembly.

ConflictSet.cpp

@@ -3260,35 +3260,28 @@ struct CheckContext {
   ConflictSet::Result *results;
   int64_t started;
   ReadContext *tls;
+#if !__has_attribute(musttail)
+  CheckJob *job;
+  bool done;
+#endif
 };
 
-#if __has_attribute(musttail)
 FLATTEN PRESERVE_NONE void keepGoing(CheckJob *job, CheckContext *context) {
+#if __has_attribute(musttail)
   job = job->next;
   MUSTTAIL return job->continuation(job, context);
+#else
+  context->job = job->next;
+  return;
+#endif
 }
-#else
-static_assert(offsetof(CheckJob, next) == 0x38);
-static_assert(offsetof(CheckJob, continuation) == 0x28);
-extern "C" void keepGoing(CheckJob *, CheckContext *);
-
-#if defined(__x86_64__)
-asm("keepGoing:\n"
-    "   mov 0x38(%rdi),%rdi\n"
-    "   jmp *0x28(%rdi)\n");
-#elif defined(__aarch64__)
-asm("keepGoing:\n"
-    "   ldr x0, [x0, #0x38]\n"
-    "   ldr x16, [x0, #0x28]\n"
-    "   br  x16\n");
-#else
-#error "Only x64 and aarch64 are supported"
-#endif
-#endif
 
 FLATTEN PRESERVE_NONE void complete(CheckJob *job, CheckContext *context) {
   if (context->started == context->count) {
     if (job->prev == job) {
+#if !__has_attribute(musttail)
+      context->done = true;
+#endif
       return;
     }
     job->prev->next = job->next;
@@ -3495,9 +3488,17 @@ struct __attribute__((visibility("hidden"))) ConflictSet::Impl {
     inProgress[0].prev = inProgress + started - 1;
     inProgress[started - 1].next = inProgress;
 
+#if __has_attribute(musttail)
     // Kick off the sequence of tail calls that finally returns once all jobs
     // are done
     inProgress->continuation(inProgress, &context);
+#else
+    context.job = inProgress;
+    context.done = false;
+    while (!context.done) {
+      context.job->continuation(context.job, &context);
+    }
+#endif
 
     for (int i = 0; i < count; ++i) {
       assert(reads[i].readVersion >= 0);