This adds support for the riscv64 architecture to the offload host
plugin. The check to define FFI_DEFAULT_ABI is intentionally not guarded
by __riscv_xlen as the value is the same for riscv32 and riscv64
(support for OpenMP on riscv32 is still under review).

Summary:
This patch adds a cache file that will automatically enable openpm,
offload, and all the fancy GPU libraries.

Summary:
We have the ability to schedule callbacks after certain events complete.
Currently we can register an arbitrary callback in CUDA, but can't in
AMDGPU. I am planning on using this support to move the RPC handling to
a separate thread, then using these callbacks to suspend / resume it
when no kernels are running. This is a preliminary patch to keep this
noise out of that one.

This does a global rename from `flang-new` to `flang`. I also
removed/changed any TODOs that I found related to making this change.

---------

Co-authored-by: H. Vetinari <h.vetinari@gmx.com>
Co-authored-by: Andrzej Warzynski <andrzej.warzynski@arm.com>

Summary:
We define this on AMDGCN but not NVPTX, which leads to some failures
dependong on the target.

We recently added versioning support to Flang's OpenMP, which restricts
and enables certain things based on the OpenMP specification version.
Currently one of the check-offload tests makes use of a feature that's
at a slightly higher version than the current default causing it to
fail.

This PR basically applies the highest current OpenMP version number as a
default argument for the lit.cfg, if we need more fine grained control
in the future we can expand it to different lit commands for each
relevant version than can then be added in each test. But for now, to
keep it simple, just set the max level version.

Removed `OmptCallbacks.cpp` since relevant contents were duplicated.
Because of the static linking there should be no change in
functionality.

Summary:
We used to do this automatically, add it back in to do it manually.