Better stack trace in MAC

Summary:
Now this gives us the real deal stack trace:

    Assertion failed: (false), function GetProperty, file db/db_impl.cc, line 4072.
    Received signal 6 (Abort trap: 6)
    #0   0x7fff57ce39b9
    #1   abort (in libsystem_c.dylib) + 125
    #2   basename (in libsystem_c.dylib) + 0
    #3   rocksdb::DBImpl::GetProperty(rocksdb::ColumnFamilyHandle*, rocksdb::Slice const&, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >*) (in db_test) (db_impl.cc:4072)
    #4   rocksdb::_Test_Empty::_Run() (in db_test) (testharness.h:68)
    #5   rocksdb::_Test_Empty::_RunIt() (in db_test) (db_test.cc:1005)
    #6   rocksdb::test::RunAllTests() (in db_test) (testharness.cc:60)
    #7   main (in db_test) (db_test.cc:6697)
    #8   start (in libdyld.dylib) + 1

Test Plan: added artificial assert, saw great stack trace

Reviewers: haobo, dhruba, ljin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18309