Please review (XS)

[steve goldman]

This fixes:

6644928 - Internal Error (src/share/vm/code/relocInfo.hpp:1089)

In the work to merge the 32/64bit assembler the assember card mark code 
was converted to use ExternalAddress to describe the base. In some cases 
the mapping of the cardtable and the heap base produce a base of zero. 
This causes an assert in the reloc code.

Testing: runThese (32bit/64bit). I was unable to reproduce the failure 
despite many tries even on the machines that saw the failures running 
the same tests so I just forced the paths for handling zero in the debugger.

http://webrev.invokedynamic.info/sgoldman/6644928/

-- 
Steve