Abstract out fetching of break_address in debug mode

If the platform has a variable length call sequence
more than simple offset math is required. This can
be true with out of line constant pools on PowerPC.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/427863003

Patch from Andrew Low <andrew_low@ca.ibm.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22935 ce2b1a6d-e550-0410-aec6-3dcde31c8c00