MIPS: Port of fix to ClassOf check from ARM.
Patch from Paul Lind <plind44@gmail.com>.

Stopped using mprotect on Cygwin.
Avoided uninitialized member warning on gcc 4.3.4
Both patches by Bert Belder.

Bug fixes and performance improvements on all platforms.

Review URL: http://codereview.chromium.org/7565005

git-svn-id: http://v8.googlecode.com/svn/trunk@8812 ce2b1a6d-e550-0410-aec6-3dcde31c8c00