Revert the latest set of platform changes.

Revert "Fix NaCl build."
Revert "Revert target arch detection."
Revert "Fix typo."
Revert "Simplify implementation of Mutex."
Revert "Fix for older clang releases that lack __has_extension."
Revert "Reland initial bits of "Implement correct OS and CC detection.""

TBR=danno@chromium.org,svenpanne@chromium.org

Review URL: https://codereview.chromium.org/21095008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15976 ce2b1a6d-e550-0410-aec6-3dcde31c8c00