Adds variables in cmake file to control the build process of drpttracer and libdrpt2ir separately. Dr's build system currently supports three modes:
(1) DR will not build drpttracer and libdrpt2ir on non-x86_64 architectures or non-Linux systems. (2) DR will build drpttracer and libdrpt2ir on x86_64 Linux systems if the submodule libipt is initialized. (3) DR will only build drpttracer on x86_64 Linux systems if libipt is not initialized.
Issue: #5505