Updated MallocHooks to collect stack traces when memory is allocated.

runtime/vm/os_fuchsia.cc

Index: runtime/vm/os_fuchsia.cc
diff --git a/runtime/vm/os_fuchsia.cc b/runtime/vm/os_fuchsia.cc
index 9c850ed4dc377ae791d99dc58299bf481d18a5ff..1a96aaa3433b7ed21fb04abaf344cd814558f420 100644
--- a/runtime/vm/os_fuchsia.cc
+++ b/runtime/vm/os_fuchsia.cc
@@ -177,6 +177,12 @@ void OS::DebugBreak() {
 }
 
 
+uintptr_t DART_NOINLINE OS::GetProgramCounter() {
+  return reinterpret_cast<uintptr_t>(
+      __builtin_extract_return_addr(__builtin_return_address(0)));
+}
+
+
 char* OS::StrNDup(const char* s, intptr_t n) {
   return strndup(s, n);
 }