Add LIBOMP_ENABLE_SHARED option for CMake

When building executables for Cray supercomputers, statically-linked executables
are preferred. This patch makes it possible to build the OpenMP runtime as an
archive for building statically-linked executables.  The patch adds the flag
LIBOMP_ENABLE_SHARED, which defaults to true. When true, a build of the OpenMP
runtime yields dynamic libraries. When false, a build of the OpenMP runtime
yields static libraries. There is no setting that allows both kinds of libraries
to be built.

Patch by John Mellor-Crummey

Differential Revision: http://reviews.llvm.org/D16525

llvm-svn: 259817