Apply name change to CMake build system.

This change has the CMake build system create a dynamic library named
libomp instead of libiomp5.  Also any reference to libiomp is replaced
with libomp.  One can still use the LIBOMP_LIB_NAME variable to enforce
a different name, and everything will still work as expected.  An important
note is that libiomp5 and libgomp symlinks are created at install time when
on Unix systems.  On Windows, copies are created with the legacy names.

llvm-svn: 238715