Change the size of the hash table used to cache IP -> debuginfo to a prime nr

This change is based on rumours/legends/oral transmission of experience/...
that prime nrs are good to use for hash table size :).

If someone has a (short) explanation about why this is useful, 
that will be welcome.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13237 a5019735-40e9-0310-863c-91ae7b9d1cf9