Followup to previous MIPS commit: implement dump(), rebase more.

When compiling with DEBUG, there is a problem linking
InstMIPS32. It overrides dump, but never defined that.

Also, update the code for some recent changes.  Namely, we
no longer check ALLOW_DUMP but instead check
BuildDefs::dump().  Also, the instruction dtors have been
deleted.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4167
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/1214863019 .