`libomp` is the default value when unconfigured in cmake, but llvm can
be configured to have `libgomp` be the default instead. Explicitly
specify this value so the test does not fail when it assumes libomp is
always the default.
Summary:
This patch reworks how we create offloading toolchains. Previously we
would handle this separately for all the different kinds. This patch
instead changes this to use the target triple and the offloading kind to
determine the proper toolchain. In the old case where the user only
passes `--offload-arch` we instead infer the triple from the passed
arguments. This is a pretty major overhaul but currently passes all the
clang tests with only minor changes to error messages.
