Create a bare bones host libbacktrace on mac.

This allows the libbacktrace library to compile for host on mac.

Bug: 12474939
Change-Id: I32cbcf996e6fe9057e5065ba21f89086343704a0
diff --git a/libbacktrace/Backtrace.cpp b/libbacktrace/Backtrace.cpp
index fa85872..20667e0 100644
--- a/libbacktrace/Backtrace.cpp
+++ b/libbacktrace/Backtrace.cpp
@@ -87,7 +87,7 @@
   if (!func_name.empty()) {
 #if defined(__APPLE__)
     // Mac OS' __cxa_demangle demangles "f" as "float"; last tested on 10.7.
-    if (symbol_name[0] != '_') {
+    if (func_name[0] != '_') {
       return func_name;
     }
 #endif