Move libbacktrace off cutils.

There's still <cutils/atomic.h> in a test, but I don't understand why
that isn't just std::atomic.

Also add a shared tgkill wrapper to libbase.

Bug: N/A
Test: ran tests
Change-Id: Idd4baa1e1670a84b3a8f35803cc5ffe5aae008a6
diff --git a/libbacktrace/Backtrace.cpp b/libbacktrace/Backtrace.cpp
index 6445a7c..6bec63c 100644
--- a/libbacktrace/Backtrace.cpp
+++ b/libbacktrace/Backtrace.cpp
@@ -23,6 +23,7 @@
 #include <string>
 
 #include <android-base/stringprintf.h>
+#include <android-base/threads.h>
 
 #include <backtrace/Backtrace.h>
 #include <backtrace/BacktraceMap.h>
@@ -31,7 +32,6 @@
 
 #include "BacktraceLog.h"
 #include "UnwindStack.h"
-#include "thread_utils.h"
 
 using android::base::StringPrintf;
 
@@ -124,7 +124,7 @@
   if (pid == BACKTRACE_CURRENT_PROCESS) {
     pid = getpid();
     if (tid == BACKTRACE_CURRENT_THREAD) {
-      tid = gettid();
+      tid = android::base::GetThreadId();
     }
   } else if (tid == BACKTRACE_CURRENT_THREAD) {
     tid = pid;