Fix compilation with recent MinGW64 versions.

Don't check for WIN32 define. Use V8_OS_* macros whenever
possible, and if not use _WIN32.

BUG=v8:2300
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/23687003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16380 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Clear next map word when folding allocations into js arrays.

BUG=
R=mstarzinger@chromium.org, mvstanton@chromium.org

Review URL: https://codereview.chromium.org/22915007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16381 ce2b1a6d-e550-0410-aec6-3dcde31c8c00