Run the verifier single-threaded.

Also improve crash dumps so the frame numbers are the same as when you're
in gdb, and add a more memorably-named environment variable for waiting
for gdb.

(cherry picked from commit 5db7ea037e9acad434efe8ab4ce798c513fdb49a)

Change-Id: I428569d84dc5728bcd090919456395be44bdc97b
diff --git a/src/compiler.cc b/src/compiler.cc
index 89f35b3..4d98d61 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -447,7 +447,11 @@
 
   TimingLogger timings("compiler");
 
+  // TODO: make the verifier thread-safe and remove this workaround.
+  size_t thread_count = thread_count_;
+  thread_count_ = 1;
   PreCompile(class_loader, dex_files, timings);
+  thread_count_ = thread_count;
 
   Compile(class_loader, dex_files);
   timings.AddSplit("Compile");