Implement a cache 'address -> symbol name' in m_debuginfo.c

Implement a cache 'address -> symbol name' in m_debuginfo.c
Support work for xtree: as xtree implementation makes a high
nr of calls to get_sym_name, this cache improves the performance
as usually, stacktraces are repeatitively querying the same
addresses.
The cache follows the same principle as the cfsi_m_cache.
In particular, cache is cleared together with the cfsi_m cache.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16121