Add simple implementation _Unwind_GetIP() for clang compiles.

Clang (prior to 3.4) does not actually provide a declaration (or definition)
of _Unwind_GetIP() for ARM. We can work around this by writing our own
basic implementation using the available primitive operations.

Change-Id: If6c66846952d8545849ad32d2b55daa4599cfe2c

libc/bionic/debug_stacktrace.cpp

@@ -80,6 +80,18 @@ struct stack_crawl_state_t {
   }
 };
 
+#if defined(__arm__) && !defined(_Unwind_GetIP)
+// Older versions of Clang don't provide a definition of _Unwind_GetIP(), so
+// we include an appropriate version of our own. Once we have updated to
+// Clang 3.4, this code can be removed.
+static __inline__
+uintptr_t _Unwind_GetIP(struct _Unwind_Context *__context) {
+  uintptr_t __ip = 0;
+  _Unwind_VRS_Get(__context, _UVRSC_CORE, 15, _UVRSD_UINT32, &__ip);
+  return __ip & ~0x1;
+}
+#endif
+
 static _Unwind_Reason_Code trace_function(__unwind_context* context, void* arg) {
   stack_crawl_state_t* state = static_cast<stack_crawl_state_t*>(arg);