[BOLT] Hash anonymous symbol names

Summary:
This diff replaces the addresses in all the {SYMBOLat,HOLEat,DATAat} symbols with hash values based on the data contained in the symbol.  It should make the profiling data for anonymous symbols robust to address changes.

The only small problem with this approach is that the hashed name for padding symbols of the same size collide frequently.  This shouldn't be a big deal since it would be weird if those symbols were hot.

On a test run with hhvm there were 26 collisions (out of ~338k symbols).  Most of the collisions were from small (2,4,8 byte) objects.

(cherry picked from FBD7134261)