If DEFAULT_SYSROOT is not specfied when building flang, then the
-isysroot flag is needed to link binaries against system libraries
on Darwin. It's also needed when linking against a non-default
sysroot.
Missed adding this check in previous commit so adding it through separate commit.
Reviewed By: raghavendhra
Differential Revision: https://reviews.llvm.org/D150626
Initial support for USE_DEVICE_PTR clause on OMP TARGET DATA directive.
Reviewed By: kiranchandramohan
Differential Revision: https://reviews.llvm.org/D148254
