Use a class field instead of global FLAG_debug_code in assember and

Use a class field instead of global FLAG_debug_code in assember and
macro assembler.

This way we can avoid changing the global flag value.

Review URL: http://codereview.chromium.org/6677044

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7178 ce2b1a6d-e550-0410-aec6-3dcde31c8c00