Fixed a bug in the chaining of fixup position

Fixed a bug in the chaining of fixup position

The ARM and MIPS assemblers had a bug where they did not handle the last element
in the list of code positions correctly during the fixup of offsets for forward
jumps. This happened when the first instruction contained a forward jump to a
label, and that label was used in a forward jump later, too.

Unified the code for Assembler::next on ARM and MIPS while we were there.

Added test cases, even for ia32/x64, which seem to be correct, even I don't
fully understand why... %-}

BUG=v8:1644
Review URL: http://codereview.chromium.org/7786001

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9063 ce2b1a6d-e550-0410-aec6-3dcde31c8c00