Change CMake variable prefix to LIBOMP

Cached CMake variables need to have a prefix so they don't collide with other
projects. This change (a lot of simple changes) simply prefixes cached variables
with LIBOMP_ and sets all of these variables to UPPERCASE which is convention.
e.g., os => LIBOMP_OS, ompt_support => LIBOMP_OMPT_SUPPORT.

Differential Revision: http://reviews.llvm.org/D9829

llvm-svn: 237845